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.

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public class Solution {
    public int firstUniqChar(String s) {
        int[] freq = new int[256];
        for(int i=0;i<s.length();i++){
            freq[s.charAt(i) - 'a']++;
        }
        for(int i=0;i<s.length();i++){
            if(freq[s.charAt(i)-'a'] == 1){
                return i;
            }
        }
        return -1;
    }
}