Minimum Window Substring

Problem

https://leetcode.com/problems/minimum-window-substring/

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 <= 10:sup:`5`

• s and t consist of uppercase and lowercase English letters.

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

Pattern

Hash Table, String, Sliding Window

Approaches

Sliding Window

Code

from collections import defaultdict


def minWindow(s: str, t: str) -> str:
    left = 0
    right = 0

    diffs = defaultdict(int)
    for char in t:
        diffs[char] -= 1

    substr = ""
    while right < len(s):
        diffs[s[right]] += 1
        right += 1

        while all([d >= 0 for d in diffs.values()]):
            if substr == "":
                substr = s[left:right]
            elif len(substr) > right - left:
                substr = s[left:right]

            diffs[s[left]] -= 1
            left += 1

    return substr

Test

>>> from minimum_window_substring__sliding_window import minWindow
>>> minWindow("ADOBECODEBANC", "ABC")
'BANC'
>>> minWindow("a", "a")
'a'
>>> minWindow("a", "aa")
''

minimum_window_substring__sliding_window.minWindow(s: str, t: str) → str