76. Minimum Window Substring

Problem Statement

Given two strings s and t of lengths m and n respectively, return the minimum window

substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

The testcases will be generated such that the answer is unique.

Example 1:

Input: s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"
Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t.

Example 2:

Input: s = "a", t = "a"
Output: "a"
Explanation: The entire string s is the minimum window.

Example 3:

Input: s = "a", t = "aa"
Output: ""
Explanation: Both 'a's from t must be included in the window.
Since the largest window of s only has one 'a', return empty string.

Constraints:

m == s.length
n == t.length
1 <= m, n <= 105
s and t consist of uppercase and lowercase English letters.

Intuition

Approach:

Basically, we need to maintain count of the t string in a map and 
maintain the same count for the s string in another map

We move the low pointer when, the count of all alphabets in window >= count of t
Shift low pointer till the condition is unmet again,

Repeat the steps

Video Links

Approach 1:

Sliding Window

C++

class Solution {
public:
    string minWindow(string S, string T) {
        string result;
        if(S.empty() || T.empty()){
            return result;
        }
        unordered_map<char, int> map;
        unordered_map<char, int> window;
        for(int i = 0; i < T.length(); i++){
            map[T[i]]++;
        }
        int minLength = INT_MAX;
        int letterCounter = 0;
        for(int slow = 0, fast = 0; fast < S.length(); fast++){
            char c = S[fast];
            if(map.find(c) != map.end()){
                window[c]++;
                if(window[c] <= map[c]){
                    letterCounter++;
                }
            }
            if(letterCounter >= T.length()){
                while(map.find(S[slow]) == map.end() || window[S[slow]] > map[S[slow]]){
                    window[S[slow]]--;
                    slow++;
                }
                if(fast - slow + 1 < minLength){
                    minLength = fast - slow + 1;
                    result = S.substr(slow, minLength);
                }
            }
        }
        return result;
    }
};

Approach 2:

C++

Approach 3:

C++

Approach 4:

C++