Leetcode Longest Substring with At Most K Distinct Characters

题意：找出最长的含k个不同字符的子串的长度。

思路：windows slide

class Solution {public:    int lengthOfLongestSubstringKDistinct(string s, int k) {        if(k < 1) return 0;                vector<int> alpha(128, 0);        int b = 0, e = 0;        int lengthMax = 0;        while(true) {             if(check(alpha, k)) { //cout << e << "  " << b << endl;                lengthMax = max(lengthMax, e - b);                if(e < s.length()) alpha[s[e]] ++;                else break;                e ++;            }            else {                alpha[s[b]] --;                b ++;            }        }                        return lengthMax;    }    private:    bool check(vector<int> alpha, int k) {        int count = 0;        for(int i = 0; i < alpha.size(); ++ i) {            if(alpha[i] != 0) count ++;        }        return count <= k;    }};