[Solved] Minimum Recolors to Get K Consecutive Black Blocks LeetCode Contest

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.

Minimum Recolors to Get K Consecutive Black Blocks LeetCode Contest

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.

Constraints:

• n == blocks.length
• 1 <= n <= 100
• blocks[i] is either 'W' or 'B'.
• 1 <= k <= n

Solution

• Sliding window of size K and minimize the no. of ‘W’ in the window
• When founding a ‘W’ in forwarding pointer, increment counter
• when found ‘W’ in the back pointer, decrement counter
• Take the minimum of the counter as the answer

  int minimumRecolors(string blocks, int k) {
        int back = 0, front = 0, count_w = 0, ans = INT_MAX;
        while(front < blocks.size()){
            if(blocks[front] == 'W'){ count_w++; }
            if(front - back + 1 == k){
                ans = min(ans, count_w);
                if(blocks[back] == 'W') count_w--;
                back++;
            }
            front++;
        }
        return ans;
    }

 public int minimumRecolors(String blocks, int k) {
        int min = Integer.MAX_VALUE;
        for (int lo = -1, hi = 0, white = 0; hi < blocks.length(); ++hi) {
            white += blocks.charAt(hi) == 'W' ? 1 : 0;
            if (hi - lo >= k) { // the window reaches size of k.
                min = Math.min(min, white); // update  minimum.
                // slide 1 step right the lower bound of the sliding
                // window and update the value of white count.
                white -= blocks.charAt(++lo) == 'W' ? 1 : 0;
            }
        }
        return min;
    }

    def minimumRecolors(self, blocks: str, k: int) -> int:
        lo, white, mi = -1, 0, inf
        for hi, c in enumerate(blocks):
            if c == 'W':
                white += 1
            if hi - lo >= k:
                mi = min(white, mi)
                lo += 1
                white -= blocks[lo] == 'W' 
        return mi

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