LeetCode First Unique Character in a String

原题链接在这里：https://leetcode.com/problems/first-unique-character-in-a-string/

题目：

Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.

题解：

扫两遍string. 第一遍s.chatAt(i)的对应count++. 第二遍找第一个count为1的char, return其index.

Time Complexity: O(s.length()). Space: O(1), 用了count array.

AC Java:

 class Solution {
     public int firstUniqChar(String s) {
         if(s == null || s.length() == 0){
             return -1;
         }

         int [] count = new int[256];
         for(int i = 0; i<s.length(); i++){
             count[s.charAt(i)]++;
         }

         for(int i = 0; i<s.length(); i++){
             if(count[s.charAt(i)] == 1){
                 return i;
             }
         }

         return -1;
     }
 }

类似Sort Characters By Frequency.