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.

class Solution {
public:
    int firstUniqChar(string s) {
        if(s.length()==0)
            return -1;
        int index[256] ;
        for(int i=0; i<256; i++)
            index[i] = -1;
        int idx = 0;
        for(int i=0; i<s.length(); i++)
        {
            if(index[s[i]]==-1)
            {
                index[s[i]] = idx;
            }else
            {
                index[s[i]] = -2;
            }
            idx++;
        }
        
        int min = INT_MAX;
        for(int i=0; i<256; i++)
        {
            if(index[i]>=0&&index[i]<min)
                min = index[i];
        }
        return min==INT_MAX? -1 : min;
    }
};