417 Pacific Atlantic Water Flow 太平洋大西洋水流

详见：https://leetcode.com/problems/pacific-atlantic-water-flow/description/

C++：

class Solution {
public:
    vector<pair<int, int>> pacificAtlantic(vector<vector<int>>& matrix) {
        if (matrix.empty() || matrix[0].empty())
        {
            return {};
        }
        vector<pair<int, int>> res;
        int m = matrix.size(), n = matrix[0].size();
        vector<vector<bool>> pacific(m, vector<bool>(n, false));
        vector<vector<bool>> atlantic(m, vector<bool>(n, false));
        for (int i = 0; i < m; ++i)
        {
            dfs(matrix, pacific, INT_MIN, i, 0);
            dfs(matrix, atlantic, INT_MIN, i, n - 1);
        }
        for (int i = 0; i < n; ++i)
        {
            dfs(matrix, pacific, INT_MIN, 0, i);
            dfs(matrix, atlantic, INT_MIN, m - 1, i);
        }
        for (int i = 0; i < m; ++i)
        {
            for (int j = 0; j < n; ++j)
            {
                if (pacific[i][j] && atlantic[i][j]) 
                {
                    res.push_back({i, j});
                }
            }
        }
        return res;
    }
    void dfs(vector<vector<int>>& matrix, vector<vector<bool>>& visited, int pre, int i, int j)
    {
        int m = matrix.size(), n = matrix[0].size();
        if (i < 0 || i >= m || j < 0 || j >= n || visited[i][j] || matrix[i][j] < pre)
        {
            return;
        }
        visited[i][j] = true;
        dfs(matrix, visited, matrix[i][j], i + 1, j);
        dfs(matrix, visited, matrix[i][j], i - 1, j);
        dfs(matrix, visited, matrix[i][j], i, j + 1);
        dfs(matrix, visited, matrix[i][j], i, j - 1);
    }
};

 参考：https://www.cnblogs.com/grandyang/p/5962508.html