Find the minimum covering substring
Return the shortest substring of source containing all characters of target with their required multiplicities. Break ties by earliest start; return an empty string if target is empty or no window exists. All input is valid JSON satisfying the constraints; do not mutate the meaning of values through type coercion. Interviewers use this problem to assess expand a window until it covers the target then shrink it greedily.
Input
A JSON array [source, target] of ASCII strings.
Output
Print one compact JSON value: a string.
Constraints
Each string has length <= 10000.
Example
Input
["ADOBECODEBANC","ABC"]
Output
"BANC"
Example 2 input
["abxxab","ab"]
Output
"ab"
Boundary case: Update the best answer only for a strictly shorter window
Public test cases (2)
Input
["ADOBECODEBANC","ABC"]
Expected output
"BANC"
Input
["aa","aa"]
Expected output
"aa"
Hints (2)
- Track missing character occurrences rather than just distinct characters
- Update the best answer only for a strictly shorter window
Target complexity: time O(n+m) · space O(1) auxiliary for fixed ASCII alphabet, excluding returned substring