leetcode 3. Longest Substring Without Repeating Characters

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int p[256];
        memset(p,-1,sizeof(p));
        int ans = 0, flag = -1;
        for(int i = 0;i < s.length();i++){
            flag = max(flag,p[s[i]]);
            p[s[i]] = i;
            ans = max(ans,i-flag);
        }
        return ans;
    }
};