2379. Minimum Recolors to Get K Consecutive Black Blocks

Method 1

You are given a 0-indexed string blocks of length n, where blocks[i] is either 'W' or 'B', representing the color of the ith block. The characters 'W' and 'B' denote the colors white and black, respectively.
You are also given an integer k, which is the desired number of consecutive black blocks.
In one operation, you can recolor a white block such that it becomes a black block.
Return the minimum number of operations needed such that there is at least one occurrence of k consecutive black blocks.

Example 1

Input: blocks = "WBBWWBBWBW", k = 7
Output: 3
Explanation:
One way to achieve 7 consecutive black blocks is to recolor the 0th, 3rd, and 4th blocks
so that blocks = "BBBBBBBWBW". 
It can be shown that there is no way to achieve 7 consecutive black blocks in less than 3 operations.
Therefore, we return 3.

Example 2

Input: blocks = "WBWBBBW", k = 2
Output: 0
Explanation:
No changes need to be made, since 2 consecutive black blocks already exist.
Therefore, we return 0.

Method 1

【O(n) time | O(1) space】

package Leetcode.SlideWindow;

/**
 * @author zhengxingxing
 * @date 2024/12/23
 */
public class MinimumRecolorsToGetKConsecutiveBlackBlocks {
    public static int minimumRecolors(String blocks, int k) {
        int n = blocks.length();
        if (n == 0) {
            return 0;
        }

        // Initialize result with maximum possible value
        int minOperations = k;

        // Count of white blocks in current window (representing required operations)
        int whiteCount = 0;

        // Process the string using sliding window
        for (int i = 0; i < n; i++) {
            // Add current element to window
            if (blocks.charAt(i) == 'W') {
                whiteCount++;
            }

            // Skip until we have enough elements (k) in our window
            if (i < k - 1) {
                continue;
            }

            // Update minimum operations needed for current window
            minOperations = Math.min(minOperations, whiteCount);

            // Remove leftmost element for next iteration
            if (blocks.charAt(i - k + 1) == 'W') {
                whiteCount--;
            }
        }

        return minOperations;
    }

    public static void main(String[] args) {

        // Test Case 1: Example from the problem
        String test1 = "WBBWWBBWBW";
        int k1 = 7;
        System.out.println("Test Case 1:");
        System.out.println("Input: blocks = \"" + test1 + "\", k = " + k1);
        System.out.println("Expected Output: 3");
        System.out.println("Actual Output: " + minimumRecolors(test1, k1));
        System.out.println();

        // Test Case 2: No changes needed
        String test2 = "WBWBBBW";
        int k2 = 2;
        System.out.println("Test Case 2:");
        System.out.println("Input: blocks = \"" + test2 + "\", k = " + k2);
        System.out.println("Expected Output: 0");
        System.out.println("Actual Output: " + minimumRecolors(test2, k2));
        System.out.println();

        // Test Case 3: All white blocks
        String test3 = "WWWWW";
        int k3 = 3;
        System.out.println("Test Case 3:");
        System.out.println("Input: blocks = \"" + test3 + "\", k = " + k3);
        System.out.println("Expected Output: 3");
        System.out.println("Actual Output: " + minimumRecolors(test3, k3));
        System.out.println();

        // Test Case 4: K equals string length
        String test4 = "WBWBW";
        int k4 = 5;
        System.out.println("Test Case 4:");
        System.out.println("Input: blocks = \"" + test4 + "\", k = " + k4);
        System.out.println("Expected Output: 3");
        System.out.println("Actual Output: " + minimumRecolors(test4, k4));
        System.out.println();
    }
}

Method 1

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