387. First Unique Character in a String

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.

Note: You may assume the string contain only lowercase letters.

解题思路为建立一个char-freq的mapping。 先将string转换为char[]，可以比string.charAt(index)运行快。

public class Solution {
    public int firstUniqChar(String s) {
        char[] array = s.toCharArray();
        int[] freq = new int[26];
        for(int i = 0; i < s.length(); i++) {
            freq[array[i] - 'a']++;
        }
        for(int i = 0; i < s.length(); i++) {
            if(freq[array[i] - 'a'] == 1) return i;
        }
        return -1;
    }
}