Sword To Offer 034 - 第一个只出现一次的字符

class Solution {
public:
    int FirstNotRepeatingChar(string str) {
        vector<int> book(256, 0);
        for(int i = 0; str[i]; i++) {
            book[str[i]]++;
        }
        for(int i = 0; str[i]; i++) {
            if(book[str[i]] == 1) {
                return i;
            }
        }
        return -1;
    }
};