K Smallest Substring

Problem Overview

• Find the substring with exactly k '1's, minimal length, and if tied, lexicographically smallest.
• Inputs: binary string input_str and integer k; Output: the chosen substring; an answer is guaranteed.
• Constraints: 1 ≤ k ≤ |s| ≤ 10^3; s[i] ∈ {'0','1'}; count('1') ≥ k.
• Context: Binary string processing and substring selection; common coding interview interview question problem.
• Source: Asked in Akuna interviews.