leetcode340 - Longest Substring with At Most K Distinct Characters - hard

Given a string, find the length of the longest substring T that contains at most k distinct characters.

Example 1:

Input: s = "eceba", k = 2
Output: 3
Explanation: T is "ece" which its length is 3.

Example 2:

Input: s = "aa", k = 1
Output: 2
Explanation: T is "aa" which its length is 2.

 

Sliding window的思路，因为要找最长的，r要尽可能拓展，同时用hashmap统计经过的char2freq，hashmap里存的distinct chars多于k了再一点点往回缩l，直到把某一个char的计数全减完，把它从hasmap里彻底删了，r再继续拓展。

 

实现：

class Solution {
public:
    int lengthOfLongestSubstringKDistinct(string s, int k) {
        
        int res = 0;
        
        unordered_map<char, int> mp;
        int l = 0, r = 0;
        while (r < s.size()){
            mp[s[r]]++;
            while (mp.size() > k){
                mp[s[l]]--;
                if (mp[s[l]] == 0)
                    mp.erase(s[l]);
                l++;
            }
            res = max(res, r-l+1);
            r++;
        }
        
        return res;
        
    }
};