题目：

题解：

type point struct{
    x, y int
}

var dirs = []point{{-1, 0}, {1, 0}, {0, -1}, {0, 1}}

func updateMatrix(mat [][]int) [][]int {
    var m, n = len(mat), len(mat[0])
    var res = make([][]int, m)
    var visited = make([][]bool, m)
    var queue []point
    for i := range mat{
        res[i] = make([]int, n)
        visited[i] = make([]bool, n)
        for j := range mat[i]{
            if mat[i][j] == 0 {
                queue = append(queue, point{i, j})
                visited[i][j] = true
            }
        }
    }
    for len(queue) != 0 {
        cur := queue[0]
        queue = queue[1:]
        for _, dir := range dirs{
            i, j := cur.x + dir.x, cur.y + dir.y
            if i >=0 && i < m && j >=0 && j < n && !visited[i][j]{
                res[i][j] = res[cur.x][cur.y] + 1
                queue = append(queue, point{i,j})
                visited[i][j] = true
            }
        }
    }
    return res
}