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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.

Leetcode Link

Examples​

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.

Code​

Python
class Solution:
    def minWindow(self, s: str, t: str) -> str:
        lookup = Counter(t)
        mx = float("inf")
        output = ""
        S = len(s)
        start, end = 0, 0
        count = len(lookup)

        while end < S:

            # End Pointer
            while end < S and count != 0:
                if s[end] in lookup:
                    lookup[s[end]] -= 1
                    if lookup[s[end]] == 0:
                        count -=1
                end += 1

            # Start Pointer
            while start <= end and count == 0:

                if end-start < mx:
                    mx = end - start
                    output = s[start:end]

                if s[start] in lookup:
                    lookup[s[start]] += 1
                    if lookup[s[start]] > 0:
                        count += 1

                start += 1

        return output