+ problem 207

fruit-in · fruit-in · commit a51a484c9623 · 2023-11-08T18:26:39.000+08:00
diff --git a/Problemset/0207-Course Schedule/README.md b/Problemset/0207-Course Schedule/README.md
@@ -0,0 +1,69 @@
+# 207. Course Schedule
+There are a total of `numCourses` courses you have to take, labeled from `0` to `numCourses - 1`. You are given an array `prerequisites` where <code>prerequisites[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> indicates that you **must** take course <code>b<sub>i</sub></code> first if you want to take course <code>a<sub>i</sub></code>.
+
+* For example, the pair `[0, 1]`, indicates that to take course `0` you have to first take course `1`.
+
+Return `true` if you can finish all courses. Otherwise, return `false`.
+
+#### Example 1:
+<pre>
+<strong>Input:</strong> numCourses = 2, prerequisites = [[1,0]]
+<strong>Output:</strong> true
+<strong>Explanation:</strong> There are a total of 2 courses to take.
+To take course 1 you should have finished course 0. So it is possible.
+</pre>
+
+#### Example 2:
+<pre>
+<strong>Input:</strong> numCourses = 2, prerequisites = [[1,0],[0,1]]
+<strong>Output:</strong> false
+<strong>Explanation:</strong> There are a total of 2 courses to take.
+To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.
+</pre>
+
+#### Constraints:
+* `1 <= numCourses <= 2000`
+* `0 <= prerequisites.length <= 5000`
+* `prerequisites[i].length == 2`
+* <code>0 <= a<sub>i</sub>, b<sub>i</sub> < numCourses</code>
+* All the pairs prerequisites[i] are **unique**.
+
+## Solutions (Rust)
+
+### 1. Solution
+```Rust
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn can_finish(num_courses: i32, prerequisites: Vec<Vec<i32>>) -> bool {
+        let mut indegrees = vec![0; num_courses as usize];
+        let mut follows = HashMap::new();
+        let mut courses = vec![];
+
+        for prerequisite in prerequisites {
+            indegrees[prerequisite[0] as usize] += 1;
+            follows
+                .entry(prerequisite[1] as usize)
+                .or_insert(vec![])
+                .push(prerequisite[0] as usize);
+        }
+
+        for b in 0..num_courses as usize {
+            if indegrees[b] == 0 {
+                courses.push(b);
+            }
+        }
+
+        while let Some(b) = courses.pop() {
+            for a in follows.remove(&b).unwrap_or(vec![]) {
+                indegrees[a] -= 1;
+                if indegrees[a] == 0 {
+                    courses.push(a);
+                }
+            }
+        }
+
+        indegrees.iter().all(|&x| x == 0)
+    }
+}
+```
diff --git a/Problemset/0207-Course Schedule/README_CN.md b/Problemset/0207-Course Schedule/README_CN.md
@@ -0,0 +1,69 @@
+# 207. 课程表
+你这个学期必须选修 `numCourses` 门课程，记为 `0` 到 `numCourses - 1` 。
+
+在选修某些课程之前需要一些先修课程。 先修课程按数组 `prerequisites` 给出，其中 <code>prerequisites[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> ，表示如果要学习课程 <code>a<sub>i</sub></code> 则 **必须** 先学习课程  <code>b<sub>i</sub></code> 。
+
+* 例如，先修课程对 `[0, 1]` 表示：想要学习课程 `0` ，你需要先完成课程 `1` 。
+
+请你判断是否可能完成所有课程的学习？如果可以，返回 `true` ；否则，返回 `false` 。
+
+#### 示例 1:
+<pre>
+<strong>输入:</strong> numCourses = 2, prerequisites = [[1,0]]
+<strong>输出:</strong> true
+<strong>解释:</strong> 总共有 2 门课程。学习课程 1 之前，你需要完成课程 0 。这是可能的。
+</pre>
+
+#### 示例 2:
+<pre>
+<strong>输入:</strong> numCourses = 2, prerequisites = [[1,0],[0,1]]
+<strong>输出:</strong> false
+<strong>解释:</strong> 总共有 2 门课程。学习课程 1 之前，你需要先完成课程 0 ；并且学习课程 0 之前，你还应先完成课程 1 。这是不可能的。
+</pre>
+
+#### 提示:
+* `1 <= numCourses <= 2000`
+* `0 <= prerequisites.length <= 5000`
+* `prerequisites[i].length == 2`
+* <code>0 <= a<sub>i</sub>, b<sub>i</sub> < numCourses</code>
+* `prerequisites[i]` 中的所有课程对 **互不相同**
+
+## 题解 (Rust)
+
+### 1. 题解
+```Rust
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn can_finish(num_courses: i32, prerequisites: Vec<Vec<i32>>) -> bool {
+        let mut indegrees = vec![0; num_courses as usize];
+        let mut follows = HashMap::new();
+        let mut courses = vec![];
+
+        for prerequisite in prerequisites {
+            indegrees[prerequisite[0] as usize] += 1;
+            follows
+                .entry(prerequisite[1] as usize)
+                .or_insert(vec![])
+                .push(prerequisite[0] as usize);
+        }
+
+        for b in 0..num_courses as usize {
+            if indegrees[b] == 0 {
+                courses.push(b);
+            }
+        }
+
+        while let Some(b) = courses.pop() {
+            for a in follows.remove(&b).unwrap_or(vec![]) {
+                indegrees[a] -= 1;
+                if indegrees[a] == 0 {
+                    courses.push(a);
+                }
+            }
+        }
+
+        indegrees.iter().all(|&x| x == 0)
+    }
+}
+```
diff --git a/Problemset/0207-Course Schedule/Solution.rs b/Problemset/0207-Course Schedule/Solution.rs
@@ -0,0 +1,34 @@
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn can_finish(num_courses: i32, prerequisites: Vec<Vec<i32>>) -> bool {
+        let mut indegrees = vec![0; num_courses as usize];
+        let mut follows = HashMap::new();
+        let mut courses = vec![];
+
+        for prerequisite in prerequisites {
+            indegrees[prerequisite[0] as usize] += 1;
+            follows
+                .entry(prerequisite[1] as usize)
+                .or_insert(vec![])
+                .push(prerequisite[0] as usize);
+        }
+
+        for b in 0..num_courses as usize {
+            if indegrees[b] == 0 {
+                courses.push(b);
+            }
+        }
+
+        while let Some(b) = courses.pop() {
+            for a in follows.remove(&b).unwrap_or(vec![]) {
+                indegrees[a] -= 1;
+                if indegrees[a] == 0 {
+                    courses.push(a);
+                }
+            }
+        }
+
+        indegrees.iter().all(|&x| x == 0)
+    }
+}
diff --git a/README.md b/README.md
@@ -140,6 +140,7 @@
 [204][204l]  |[Count Primes][204]                                                                   |![rs]
 [205][205l]  |[Isomorphic Strings][205]                                                             |![py]
 [206][206l]  |[Reverse Linked List][206]                                                            |![py]
+[207][207l]  |[Course Schedule][207]                                                                |![rs]
 [208][208l]  |[Implement Trie (Prefix Tree)][208]                                                   |![py]
 [209][209l]  |[Minimum Size Subarray Sum][209]                                                      |![rs]
 [211][211l]  |[Design Add and Search Words Data Structure][211]                                     |![py]
@@ -1410,6 +1411,7 @@
 [204]:Problemset/0204-Count%20Primes/README.md#204-count-primes
 [205]:Problemset/0205-Isomorphic%20Strings/README.md#205-isomorphic-strings
 [206]:Problemset/0206-Reverse%20Linked%20List/README.md#206-reverse-linked-list
+[207]:Problemset/0207-Course%20Schedule/README.md#207-course-schedule
 [208]:Problemset/0208-Implement%20Trie%20\(Prefix%20Tree\)/README.md#208-implement-trie-prefix-tree
 [209]:Problemset/0209-Minimum%20Size%20Subarray%20Sum/README.md#209-minimum-size-subarray-sum
 [211]:Problemset/0211-Design%20Add%20and%20Search%20Words%20Data%20Structure/README.md#211-design-add-and-search-words-data-structure
@@ -2675,6 +2677,7 @@
 [204l]:https://leetcode.com/problems/count-primes/
 [205l]:https://leetcode.com/problems/isomorphic-strings/
 [206l]:https://leetcode.com/problems/reverse-linked-list/
+[207l]:https://leetcode.com/problems/course-schedule/
 [208l]:https://leetcode.com/problems/implement-trie-prefix-tree/
 [209l]:https://leetcode.com/problems/minimum-size-subarray-sum/
 [211l]:https://leetcode.com/problems/add-and-search-word-data-structure-design/
diff --git a/README_CN.md b/README_CN.md
@@ -140,6 +140,7 @@
 [204][204l]  |[计数质数][204]                                           |![rs]
 [205][205l]  |[同构字符串][205]                                         |![py]
 [206][206l]  |[反转链表][206]                                           |![py]
+[207][207l]  |[课程表][207]                                             |![rs]
 [208][208l]  |[实现 Trie (前缀树)][208]                                 |![py]
 [209][209l]  |[长度最小的子数组][209]                                   |![rs]
 [211][211l]  |[添加与搜索单词 - 数据结构设计][211]                      |![py]
@@ -1410,6 +1411,7 @@
 [204]:Problemset/0204-Count%20Primes/README_CN.md#204-计数质数
 [205]:Problemset/0205-Isomorphic%20Strings/README_CN.md#205-同构字符串
 [206]:Problemset/0206-Reverse%20Linked%20List/README_CN.md#206-反转链表
+[207]:Problemset/0207-Course%20Schedule/README_CN.md#207-课程表
 [208]:Problemset/0208-Implement%20Trie%20\(Prefix%20Tree\)/README_CN.md#208-实现-trie-前缀树
 [209]:Problemset/0209-Minimum%20Size%20Subarray%20Sum/README_CN.md#209-长度最小的子数组
 [211]:Problemset/0211-Design%20Add%20and%20Search%20Words%20Data%20Structure/README_CN.md#211-添加与搜索单词---数据结构设计
@@ -2675,6 +2677,7 @@
 [204l]:https://leetcode.cn/problems/count-primes/
 [205l]:https://leetcode.cn/problems/isomorphic-strings/
 [206l]:https://leetcode.cn/problems/reverse-linked-list/
+[207l]:https://leetcode.cn/problems/course-schedule/
 [208l]:https://leetcode.cn/problems/implement-trie-prefix-tree/
 [209l]:https://leetcode.cn/problems/minimum-size-subarray-sum/
 [211l]:https://leetcode.cn/problems/add-and-search-word-data-structure-design/
