0076: 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.

Follow up: Could you find an algorithm that runs in O(m + n) time?

Code Solution

from collections import Counter

class Solution:
    def minWindow(self, s, t):
        
        tgt = Counter(t)
        
        lx = 0
        src = {}
        cmin = 1e9
        out = ""
        for rx, c in enumerate(s):
            
            src[c] = src.get(c, 0) + 1
            
            while all([src.get(cc, 0) >= tgt[cc] for cc in t]):
                if cmin > rx - lx + 1:
                    cmin = rx - lx + 1
                    out = s[lx:rx + 1]
                src[s[lx]] -= 1
                lx = lx + 1
        
        return out