Longest Substring Without Repeating Characters 7 lines with c++ in 12ms

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int m = 0;
        for (int i = 0, len = s.length(), l = 0, p[256] = { 0 }, c; i < len; ) {
            l = max(l, p[c = s.at(i), c]);
            p[c] = ++i;
            m = max(m, i - l);
        }
        return m;
    }
};