417. Pacific Atlantic Water Flow

Given an m x n matrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges.

Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower.

Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean.

Note:

1. The order of returned grid coordinates does not matter.
2. Both m and n are less than 150.

Example:

Given the following 5x5 matrix:

  Pacific ~   ~   ~   ~   ~
       ~  1   2   2   3  (5) *
       ~  3   2   3  (4) (4) *
       ~  2   4  (5)  3   1  *
       ~ (6) (7)  1   4   5  *
       ~ (5)  1   1   2   4  *
          *   *   *   *   * Atlantic

Return:

[[0, 4], [1, 3], [1, 4], [2, 2], [3, 0], [3, 1], [4, 0]] (positions with parentheses in above matrix).

# @lc code=start
using LeetCode

function pacific_atlantic(heights::Matrix{Int})
    r, c = size(heights)
    visited = fill(false, r, c)
    res = fill(0x0, r, c)
    res[1:end, 1] .= 0x1
    res[1, 1:end] .= 0x1
    res[end, 1:end] .= 0x2
    res[1:end, end] .= 0x2
    res[1, end] = res[end, 1] = 0x3
    dirs = [(1, 0), (-1, 0), (0, 1), (0, -1)]

    function dfs(x, y)
        visited[x, y] = true
        for dir in dirs
            nx, ny = x + dir[1], y + dir[2]
            (0 < nx <= r && 0 < ny <= c) || continue
            if heights[x, y] <= heights[nx, ny]
                res[nx, ny] |= res[x, y]
                !visited[nx, ny] && dfs(nx, ny)
            end
        end
    end
    for i in 1:r
        dfs(i, 1), dfs(i, c)
    end
    for j in 1:c
        dfs(1, j), dfs(r, j)
    end
    r = Set{Tuple{Int, Int}}()
    for cidx in CartesianIndices(res)
        res[cidx] == 0x3 && push!(r, cidx.I)
    end
    return r
end
# @lc code=end

pacific_atlantic (generic function with 1 method)

This page was generated using DemoCards.jl and Literate.jl.