https://leetcode.cn/problems/trapping-rain-water-ii/

func trapRainWater(heightMap [][]int) int {
    m, n := len(heightMap), len(heightMap[0])
    maxHeight := 0
    for _, row := range heightMap {//找到最高的格子
        for _, h := range row {
            maxHeight = max(maxHeight, h)
        }
    }

    water := make([][]int,m)
    for i := range water {//初始化每个位置的高度都为最大
        water[i] = make([]int, n)
        for j := range water[i] {
            water[i][j] = maxHeight
        }
    }
    type xy struct{ x, y int }
    q := []xy{} //bfs队列
    for i := range heightMap {
        for j:= range heightMap[i] {
            if (i == 0 || i == m-1 || j == 0 || j == n-1) && heightMap[i][j] < water[i][j] {
                water[i][j] = heightMap[i][j]//最外层方块不能接水
                q = append(q, xy{i, j}) //bfs入口，从最外层开始搜索
            }
        }
    }

    dirs := []int{-1, 0, 1, 0, -1}//四方向搜索
    for len(q) > 0 {
        p := q[0]
        q = q[1:]
        x, y := p.x, p.y
        for i := 0; i < 4; i++ {
            nx, ny := x+dirs[i], y+dirs[i+1]
            //1、位置合法 2、搜索位置比当前维护的最大高度要高 3、搜索位置比当前位置要高
            if 0 <= nx && nx < m && 0 <= ny && ny < n && water[nx][ny] > water[x][y] && water[nx][ny] > heightMap[nx][ny] {
                water[nx][ny] = max(water[x][y], heightMap[nx][ny])
                q = append(q, xy{nx, ny})
            }
        }
    }
    ans:=0
    for i := range heightMap {
        for j := range heightMap[i] {
            ans += water[i][j] - heightMap[i][j]
        }
    }
    return ans
}


func max(a, b int) int {
    if b > a {
        return b
    }
    return a
}