76. Minimum Window Substring

Problem Link

Description

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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?

Solution

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Python3

class Solution:
    def minWindow(self, s: str, t: str) -> str:
        N, M = len(s), len(t)
        res = (-1, inf) # (start, length)
        counter = Counter(t)
        curr = len(counter)
        i = 0
 
        for j, x in enumerate(s):
            if x in counter:
                counter[x] -= 1
                if counter[x] == 0:
                    curr -= 1
            
            while curr == 0:
                length = j - i + 1
                if length < res[1]:
                    res = (i, length)
                
                if s[i] in counter:
                    counter[s[i]] += 1
                    if counter[s[i]] == 1:
                        curr += 1
                
                i += 1
        
        start, length = res
        if start == -1: return ""
 
        return s[start : start + length]