2379. Minimum Recolors to Get K Consecutive Black Blocks

EasyView on LeetCode

String Sliding Window

Time: O(n)

Space: O(1)

Intuition

Count black cells after painting rows and columns. Inclusion-exclusion: |rows| * m + |cols| * n - |rows| * |cols|.

Algorithm

1Count unique rows and unique cols from coordinates.
2Answer = uniqueRows * n + uniqueCols * m - uniqueRows * uniqueCols.

Common Pitfalls

•Intersection cells (row and col both painted) are counted twice without subtraction.

2379.cs

// Approach: Sliding window of size k; count max 'B's in any window; answer = k - maxB.
// Time: O(n) Space: O(1)

public class Solution
{
    public int MinimumRecolors(string blocks, int k)
    {
        int countB = 0;
        int maxCountB = 0;

        for (int i = 0; i < blocks.Length; ++i)
        {
            if (blocks[i] == 'B')
                ++countB;
            if (i >= k && blocks[i - k] == 'B')
                --countB;
            maxCountB = Math.Max(maxCountB, countB);
        }

        return k - maxCountB;
    }
}

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