add 最长公共前缀

Author: ScoutYin

README.md

@@ -8,6 +8,7 @@
 - [Two Sum](https://github.com/ScoutYin/LeetCode-JavaScript/blob/master/solutions/easy/twoSum.md)
 - [Search Insert Position](https://github.com/ScoutYin/LeetCode-JavaScript/blob/master/solutions/easy/searchInsert.md)
 - [Count And Say](https://github.com/ScoutYin/LeetCode-Javascript/blob/master/solutions/easy/countAndSay.md)
+- [最长公共前缀](https://github.com/ScoutYin/LeetCode-Javascript/blob/master/solutions/easy/longestCommonPrefix.md)
 
 ### Medium
 

solutions/easy/longestCommonPrefix.md

@@ -0,0 +1,54 @@
+## 最长公共前缀
+
+> [领扣-最长公共前缀](https://leetcode-cn.com/problems/longest-common-prefix/description/)
+
+### Question
+
+编写一个函数来查找字符串数组中的最长公共前缀。
+
+如果不存在公共前缀，返回空字符串 ""。
+
+所有输入只包含小写字母 a-z 。
+
+#### Example:
+```shell
+输入：["flower","flow","flight"]
+输出："fl"
+```
+
+```shell
+输入: ["dog","racecar","car"]
+输出: ""
+解释: 输入不存在公共前缀。
+```
+
+### Solution
+```javascript
+/**
+ * @param {string[]} strs
+ * @return {string}
+ */
+const longestCommonPrefix = (strs) => {
+  if (!strs.length || strs[0] === '') return ''
+  if (strs.length === 1) return strs[0]
+  let index = 1
+  let last = ''
+  let prefix = ''
+  const check = () => {
+    prefix = strs[0].substring(0, index)
+    if (last === prefix) return
+    const isCommon = strs.every(str => str.indexOf(prefix) === 0) 
+    if (isCommon) {
+      last = prefix
+      index++
+      check()
+    }
+  }
+  check()
+  return last
+}
+```
+
+### 需注意的的几种情况
+
+输入：`[]`、`[""]`、`["a"]`、`["a", "a"]`