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‎Problemset/0880-Decoded String at Index/README.md

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# 880. Decoded String at Index
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You are given an encoded string `s`. To decode the string to a tape, the encoded string is read one character at a time and the following steps are taken:
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* If the character read is a letter, that letter is written onto the tape.
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* If the character read is a digit `d`, the entire current tape is repeatedly written `d - 1` more times in total.
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Given an integer `k`, return *the* <code>k<sup>th</sup></code> *letter (**1-indexed**) in the decoded string*.
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#### Example 1:
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<pre>
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<strong>Input:</strong> s = "leet2code3", k = 10
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<strong>Output:</strong> "o"
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<strong>Explanation:</strong> The decoded string is "leetleetcodeleetleetcodeleetleetcode".
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The 10th letter in the string is "o".
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</pre>
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#### Example 2:
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<pre>
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<strong>Input:</strong> s = "ha22", k = 5
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<strong>Output:</strong> "h"
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<strong>Explanation:</strong> The decoded string is "hahahaha".
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The 5th letter is "h".
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</pre>
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#### Example 3:
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<pre>
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<strong>Input:</strong> s = "a2345678999999999999999", k = 1
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<strong>Output:</strong> "a"
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<strong>Explanation:</strong> The decoded string is "a" repeated 8301530446056247680 times.
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The 1st letter is "a".
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</pre>
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#### Constraints:
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* `2 <= s.length <= 100`
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* `s` consists of lowercase English letters and digits `2` through `9`.
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* `s` starts with a letter.
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* <code>1 <= k <= 10<sup>9</sup></code>
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* It is guaranteed that `k` is less than or equal to the length of the decoded string.
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* The decoded string is guaranteed to have less than <code>2<sup>63</sup></code> letters.
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## Solutions (Rust)
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### 1. Solution
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```Rust
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impl Solution {
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pub fn decode_at_index(s: String, k: i32) -> String {
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let mut k = k as i64 - 1;
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let mut chars = vec![];
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let mut length = 0;
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for ch in s.bytes() {
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chars.push((ch, length));
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if ch.is_ascii_lowercase() {
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length += 1;
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} else {
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length *= (ch - b'0') as i64;
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}
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if length > k {
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break;
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}
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}
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while let Some((ch, i)) = chars.pop() {
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if ch.is_ascii_lowercase() {
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if i == k {
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return String::from_utf8(vec![ch]).unwrap();
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}
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length -= 1;
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} else {
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length /= (ch - b'0') as i64;
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k %= length;
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}
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}
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unreachable!()
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}
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}
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```

‎Problemset/0880-Decoded String at Index/README_CN.md

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# 880. 索引处的解码字符串
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给定一个编码字符串 `S`。请你找出 **解码字符串** 并将其写入磁带。解码时，从编码字符串中 **每次读取一个字符** ，并采取以下步骤：
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* 如果所读的字符是字母，则将该字母写在磁带上。
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* 如果所读的字符是数字（例如 `d`），则整个当前磁带总共会被重复写 `d-1` 次。
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现在，对于给定的编码字符串 `S` 和索引 `K`，查找并返回解码字符串中的第 `K` 个字母。
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#### 示例 1:
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<pre>
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<strong>输入:</strong> s = "leet2code3", k = 10
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<strong>输出:</strong> "o"
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<strong>解释:</strong> 解码后的字符串为 "leetleetcodeleetleetcodeleetleetcode"。
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字符串中的第 10 个字母是 "o"。
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</pre>
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#### 示例 2:
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<pre>
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<strong>输入:</strong> s = "ha22", k = 5
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<strong>输出:</strong> "h"
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<strong>解释:</strong> 解码后的字符串为 "hahahaha"。第 5 个字母是 "h"。
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</pre>
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#### 示例 3:
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<pre>
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<strong>输入:</strong> s = "a2345678999999999999999", k = 1
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<strong>输出:</strong> "a"
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<strong>解释:</strong> 解码后的字符串为 "a" 重复 8301530446056247680 次。第 1 个字母是 "a"。
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</pre>
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#### 提示:
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* `2 <= S.length <= 100`
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* `S` 只包含小写字母与数字 `2` 到 `9` 。
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* `S` 以字母开头。
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* <code>1 <= k <= 10<sup>9</sup></code>
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* 题目保证 `K` 小于或等于解码字符串的长度。
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* 解码后的字符串保证少于 <code>2<sup>63</sup></code> 个字母。
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## 题解 (Rust)
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### 1. 题解
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```Rust
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impl Solution {
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pub fn decode_at_index(s: String, k: i32) -> String {
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let mut k = k as i64 - 1;
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let mut chars = vec![];
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let mut length = 0;
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for ch in s.bytes() {
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chars.push((ch, length));
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if ch.is_ascii_lowercase() {
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length += 1;
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} else {
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length *= (ch - b'0') as i64;
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}
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if length > k {
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break;
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}
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}
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while let Some((ch, i)) = chars.pop() {
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if ch.is_ascii_lowercase() {
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if i == k {
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return String::from_utf8(vec![ch]).unwrap();
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}
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length -= 1;
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} else {
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length /= (ch - b'0') as i64;
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k %= length;
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}
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}
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unreachable!()
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}
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}
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```

‎Problemset/0880-Decoded String at Index/Solution.rs

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impl Solution {
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pub fn decode_at_index(s: String, k: i32) -> String {
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let mut k = k as i64 - 1;
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let mut chars = vec![];
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let mut length = 0;
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for ch in s.bytes() {
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chars.push((ch, length));
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if ch.is_ascii_lowercase() {
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length += 1;
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} else {
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length *= (ch - b'0') as i64;
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}
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if length > k {
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break;
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}
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}
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while let Some((ch, i)) = chars.pop() {
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if ch.is_ascii_lowercase() {
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if i == k {
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return String::from_utf8(vec![ch]).unwrap();
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}
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length -= 1;
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} else {
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length /= (ch - b'0') as i64;
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k %= length;
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}
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}
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unreachable!()
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}
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}

‎README.md

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[876][876l] |[Middle of the Linked List][876] |![py]
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[877][877l] |[Stone Game][877] |![rs]
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[878][878l] |[Nth Magical Number][878] |![py]
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[880][880l] |[Decoded String at Index][880] |![rs]
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[881][881l] |[Boats to Save People][881] |![rb]
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[883][883l] |[Projection Area of 3D Shapes][883] |![rs]
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[884][884l] |[Uncommon Words from Two Sentences][884] |![py]
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[876]:Problemset/0876-Middle%20of%20the%20Linked%20List/README.md#876-middle-of-the-linked-list
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[877]:Problemset/0877-Stone%20Game/README.md#877-stone-game
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[878]:Problemset/0878-Nth%20Magical%20Number/README.md#878-nth-magical-number
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[880]:Problemset/0880-Decoded%20String%20at%20Index/README.md#880-decoded-string-at-index
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[881]:Problemset/0881-Boats%20to%20Save%20People/README.md#881-boats-to-save-people
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[883]:Problemset/0883-Projection%20Area%20of%203D%20Shapes/README.md#883-projection-area-of-3d-shapes
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[884]:Problemset/0884-Uncommon%20Words%20from%20Two%20Sentences/README.md#884-uncommon-words-from-two-sentences
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[876l]:https://leetcode.com/problems/middle-of-the-linked-list/
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[877l]:https://leetcode.com/problems/stone-game/
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[878l]:https://leetcode.com/problems/nth-magical-number/
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[880l]:https://leetcode.com/problems/decoded-string-at-index/
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[881l]:https://leetcode.com/problems/boats-to-save-people/
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[883l]:https://leetcode.com/problems/projection-area-of-3d-shapes/
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[884l]:https://leetcode.com/problems/uncommon-words-from-two-sentences/

‎README_CN.md

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[876][876l] |[链表的中间结点][876] |![py]
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[877][877l] |[石子游戏][877] |![rs]
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[878][878l] |[第 N 个神奇数字][878] |![py]
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[880][880l] |[索引处的解码字符串][880] |![rs]
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[881][881l] |[救生艇][881] |![rb]
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[883][883l] |[三维形体投影面积][883] |![rs]
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[884][884l] |[两句话中的不常见单词][884] |![py]
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[876]:Problemset/0876-Middle%20of%20the%20Linked%20List/README_CN.md#876-链表的中间结点
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[877]:Problemset/0877-Stone%20Game/README_CN.md#877-石子游戏
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[878]:Problemset/0878-Nth%20Magical%20Number/README_CN.md#878-第-n-个神奇数字
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[880]:Problemset/0880-Decoded%20String%20at%20Index/README_CN.md#880-索引处的解码字符串
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[881]:Problemset/0881-Boats%20to%20Save%20People/README_CN.md#881-救生艇
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[883]:Problemset/0883-Projection%20Area%20of%203D%20Shapes/README_CN.md#883-三维形体投影面积
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[884]:Problemset/0884-Uncommon%20Words%20from%20Two%20Sentences/README_CN.md#884-两句话中的不常见单词
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[876l]:https://leetcode.cn/problems/middle-of-the-linked-list/
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[877l]:https://leetcode.cn/problems/stone-game/
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[878l]:https://leetcode.cn/problems/nth-magical-number/
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[880l]:https://leetcode.cn/problems/decoded-string-at-index/
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[881l]:https://leetcode.cn/problems/boats-to-save-people/
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[883l]:https://leetcode.cn/problems/projection-area-of-3d-shapes/
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[884l]:https://leetcode.cn/problems/uncommon-words-from-two-sentences/

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