Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

class Solution {//easy
public:
    int lengthOfLongestSubstring(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> charPos(300,-1);//latest pos of a letter
        int start = 0, max = 0;
        for(int end = 0; end < s.size(); ++end){
            char const letter = s[end];
            if (charPos[letter] >= start){//letter exists
                start = charPos[letter] + 1;
            }
            charPos[letter] = end;
            int const length = end-start+1;
            if (max < length) max=length;
        }
        return max;
    }
};