Minimum Window Substring in Swift

You are given two strings s and t made of English letters. Return the shortest substring of s that contains every character of t, repeats included: if a letter appears twice in t, the substring must contain it at least twice.

If no substring of s contains all the characters of t, return an empty string, "". Uppercase and lowercase letters are different characters.

A substring is a run of consecutive characters of s. The tests are made so that the answer is unique.

Example 1:

Input: s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"
Explanation: "BANC" contains an A, a B and a C, and no shorter substring contains all three. "ADOBEC" and "CODEBA" contain them too, but they are longer.

Example 2:

Input: s = "a", t = "a"
Output: "a"
Explanation: s itself contains every character of t, and there is nothing shorter to try.

Example 3:

Input: s = "a", t = "aa"
Output: ""
Explanation: t needs two a's, but s has only one, so no substring works. The expected output shows an empty line for it.

Constraints:

• 1 <= s.length, t.length <= 105
• s and t are made of uppercase and lowercase English letters only (a to z, A to Z).