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Minimum Window Substring

hard
Blind 75NeetCode 150stringssliding-windowhashing

Problem

Given a string s and a pattern t, find the shortest contiguous substring of s that contains every character of t including duplicates. If none exists, return an empty string.

Examples

Input: s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"
The shortest window covering A, B, and C.
Input: s = "a", t = "aa"
Output: ""
s lacks a second a.

Constraints

  • • 1 <= s.length, t.length <= 10^5
  • • Strings contain upper and lower case English letters
Hints(tap to reveal)
  • 1. Track how many required characters are still missing.
  • 2. Expand to satisfy, then contract to minimize.
Optimal approach(spoiler)
  1. Count the required character frequencies from t.
  2. Expand the right edge, decrementing needs and a missing counter.
  3. Once all needs are met, contract the left edge while still valid.
  4. Record the smallest valid window seen.
  5. O(n) time, O(alphabet) space.

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