LeetCode 387. 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.