387. First Unique Character in a String

class Solution {
public:
    int firstUniqChar(string s) {
        int length = s.size();
        if(length <= 0)
            return -1;
        vector<int> result(256);
        for(int i = 0;i < 256;i++)
            result[i] = -1;
        for(int i = 0;i < length;i++){
            if(result[s[i]] == -1)
                result[s[i]] = i;
            else
                result[s[i]] = -2;
        }
        int min_num = 0x7FFFFFFF;
        for(int i = 0;i < 256;i++){
            if(result[i] < min_num && result[i] >= 0)
                min_num = result[i];
        }
        if(min_num == 0x7FFFFFFF)
            min_num = -1;
        return min_num;
    }
};