LeetCode1234. Replace the Substring for Balanced String

一、题目

You are given a string s of length n containing only four kinds of characters: ‘Q’, ‘W’, ‘E’, and ‘R’.

A string is said to be balanced if each of its characters appears n / 4 times where n is the length of the string.

Return the minimum length of the substring that can be replaced with any other string of the same length to make s balanced. If s is already balanced, return 0.

Example 1:

Input: s = “QWER”
Output: 0
Explanation: s is already balanced.
Example 2:

Input: s = “QQWE”
Output: 1
Explanation: We need to replace a ‘Q’ to ‘R’, so that “RQWE” (or “QRWE”) is balanced.
Example 3:

Input: s = “QQQW”
Output: 2
Explanation: We can replace the first “QQ” to “ER”.

Constraints:

n == s.length
4 <= n <= 105
n is a multiple of 4.
s contains only ‘Q’, ‘W’, ‘E’, and ‘R’.

二、题解

滑动窗口，如果l-r上可以随便填，是否能满足要求，遍历找最小的窗口（l和r都只会向右移动）

class Solution {
public:
    int balancedString(string s) {
        int n = s.size(),res = n;
        int req = n / 4;
        vector<int> arr(n,0);
        vector<int> cnt(4,0);
        for(int i = 0;i < n;i++){
            if(s[i] == 'Q') arr[i] = 0;
            else if(s[i] == 'W') arr[i] = 1;
            else if(s[i] == 'E') arr[i] = 2;
            else arr[i] = 3;
            cnt[arr[i]]++;
        }
        for(int l = 0,r = 0;l < n;l++){
            while(!ok(cnt,r-l,req) && r < n) cnt[arr[r++]]--;
            if(ok(cnt,r-l,req)) res = min(res,r-l);
            cnt[arr[l]]++;
        }
        return res;
    }
    bool ok(vector<int> cnt,int len,int req){
        for(int i = 0;i < 4;i++){
            if(cnt[i] > req) return false;
            len -= req - cnt[i];
        }
        return len == 0;
    }
};