0542 01 Matrix

class Solution {
    func updateMatrix(_ matrix: [[Int]]) -> [[Int]] {
        // fill the zeros first

        var mat = matrix
        var queue: [(Int, Int)] = []

        for (i, row) in mat.enumerated() {
            for (j, col) in row.enumerated() {
                if mat[i][j] == 0 {
                    queue.append((i, j))
                }
                mat[i][j] = -1
            }
        }

        var level = 0

        while !queue.isEmpty {
            let count = queue.count
            for i in 0..<count {
                let (r, c) = queue.remove(at: 0)
                if mat[r][c] == -1 {
                    mat[r][c] = level
                }
                if r > 0 && mat[r-1][c] == -1 {
                    queue.append((r-1, c))
                }
                if c > 0 && mat[r][c-1] == -1 {
                    queue.append((r, c-1))
                }
                if r < mat.count - 1 && mat[r+1][c] == -1 {
                    queue.append((r+1, c))
                }
                if c < mat[0].count - 1 && mat[r][c+1] == -1 {
                    queue.append((r, c+1))
                }
            }
            level += 1
        }
        return mat
    }
}
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