Create 387. First Unique Character in a String.md

387. First Unique Character in a String.md

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+# [387. First Unique Character in a String](https://leetcode.com/problems/first-unique-character-in-a-string/description/)
+# 思路
+先遍历一遍字符串，记录每个字符出现的次数，再遍历一遍找出第一个没有重复的字符。   
+由于题目说了全部字符都是小写字母，所以开辟一个大小为26的数组记录字符出现次数即可。   
+时间复杂度O(n)   
+# C++
+```
+class Solution {
+public:
+    int firstUniqChar(string s) {
+        vector<int>count(26, 0);
+        for(int i = 0; i < s.size(); i++) count[s[i] - 'a']++;
+        for(int i = 0; i < s.size(); i++) 
+            if(count[s[i] - 'a'] == 1) return i;
+        return -1;
+    }
+};
+```