Shortest Substring Containing Characters

You're given two strings, s and t. Return the shortest contiguous slice of s that contains every character of t, counting multiplicities — if t is "AABC", the slice must hold at least two As, one B, and one C. Order inside the slice doesn't matter; only the characters and their counts do. Return '' if no such slice exists. This is LeetCode 76, "Minimum Window Substring", and the canonical example is minWindow("ADOBECODEBANC", "ABC") → "BANC".

The slice must be a real substring of s (a slice()-style range), not a re-arrangement of characters from s. If multiple smallest windows exist, return the one whose start index is smallest.

Signature

function stringShortestSubstringContainingCharacters(
  s: string,
  t: string,
): string;

Examples

// LeetCode classic — "BANC" is the smallest slice covering A, B, and C.
stringShortestSubstringContainingCharacters('ADOBECODEBANC', 'ABC'); // 'BANC'

// No window — 's' contains no 'z'.
stringShortestSubstringContainingCharacters('hello world', 'z'); // ''

// Exact match — the only window that covers t is s itself.
stringShortestSubstringContainingCharacters('abc', 'abc'); // 'abc'

// t longer than s — impossible.
stringShortestSubstringContainingCharacters('abc', 'abcd'); // ''

// Repeated characters in t — the window must hold TWO A's and one C.
stringShortestSubstringContainingCharacters('AABBC', 'AAC'); // 'AABBC'

Notes

• Multiplicities matter. If t has two 'A's, the answer must hold at least two 'A's. A window with one 'A' does not satisfy 'AA'.
• Order does not matter. The characters of t may appear in any order inside the window, with anything between them.
• Case-sensitive. 'A' and 'a' are different characters. Do not lowercase the inputs.
• Empty inputs. If t is empty or longer than s, no window is possible — return ''. The same applies if s is empty.
• Ties. When two windows tie on length, return the one that starts earlier in s.
• Don't worry about Unicode astral characters (emoji, etc.). The tests use BMP characters only.
• Don't mutate the inputs. Strings are primitives in JavaScript, but the contract still says the return value is a fresh substring carved out of s, not a reconstruction.