The Minimum Recolors to Get K Consecutive Black Blocks problem gives you a string of 'W' (white) and 'B' (black) blocks. You need to find the minimum number of white blocks you must recolor to black to create at least one window of exactly K consecutive black blocks. This Minimum Recolors to Get K Consecutive Black Blocks coding problem is a clean, beginner-friendly introduction to the fixed-size sliding window technique.
Companies like Meta, Amazon, Google, and Dailyhunt use this as a warm-up problem to assess familiarity with the sliding window interview pattern. It's deceptively simple but confirms that candidates understand windowed counting, a foundational skill for harder string and array problems. The sliding window and string interview pattern is directly applied here.
The pattern is a fixed-size sliding window. Maintain a window of size K and count the number of 'W' characters inside it — that's how many recolors are needed to make those K blocks all black. Slide the window one step at a time, adding the new character and removing the outgoing one, updating the 'W' count accordingly. Track the minimum 'W' count across all windows.
String: "WBBWWBBWB", K = 3.
Fixed-size sliding window problems follow a standard template: initialize, slide, track optimum. Memorize this pattern because it's the basis for dozens of interview questions. After solving this, tackle problems with variable window sizes to deepen your understanding. The transition from fixed to variable windows — where you grow/shrink based on a condition — is the next step in mastering the sliding window interview pattern.
