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.

class Solution {
public:
    int firstUniqChar(string s) {
        int arr['z'+1] = {0};
        for(int i=0;i