[LeetCode C++/Python3] 76. Minimum Window Substring

Python3 Solution:

from collections import defaultdict
class Solution:
    def minWindow(self, s: str, t: str) -> str:
        need = defaultdict(int)
        window = defaultdict(int)
        for ch in t:
            need[ch] += 1
        
        left, right = 0, 0
        valid = 0
        
        start, nlen = 0, float('inf')
        while right < len(s):
            ch = s[right]
            right += 1
            
            if ch in need:
                window[ch] += 1
                if window[ch] == need[ch]:
                    valid += 1
                    
            while valid == len(need):
                if right - left < nlen:
                    start = left
                    nlen = right - left
                
                ch = s[left]
                left += 1
                if ch in need:
                    if window[ch] == need[ch]:
                        valid -= 1
                    window[ch] -= 1
        
        return "" if nlen == float('inf') else s[start:start+nlen]

C++ Solution:

class Solution {
public:
    string minWindow(string s, string t) {
        unordered_map<char, int> need, window;
        for(auto c:t)
            need[c]++;
        
        int left = 0, right = 0;
        int valid = 0;
        int start = 0, len = INT_MAX;
        
        while(right < s.size()){
            char ch = s[right];
            right++;
            if(need.count(ch)){
                window[ch]++;
                if(window[ch] == need[ch])
                    valid++;
            }
            while(valid == need.size()){
                if(right - left < len){
                    start = left;
                    len = right - left;                
                }

                ch = s[left];
                left++;
                if(need.count(ch)){
                    if(window[ch] == need[ch])
                        valid--;
                    window[ch]--;
                }
            }
        } 
        return len == INT_MAX ? "" : s.substr(start, len);
    }
};