417. 太平洋大西洋水流问题 leetcode

思路:从四个边界，逆流往上寻找标记的思路

class Solution {
public:
    vector<vector<int>> pacificAtlantic(vector<vector<int>>& matrix) {
        vector<vector<int>> res;
        if(matrix.empty()) return res;
        m = matrix.size();
        n = matrix[0].size();
        vector<vector<bool>> Pacificmark(m,vector<bool>(n,false));
        vector<vector<bool>> Atlanticmark(m,vector<bool>(n,false));

        for(int i=0;i<m;i++){
            dfs(matrix,Pacificmark,i,0);
            dfs(matrix,Atlanticmark,i,n-1);
        }

        for(int i=0;i<n;i++){
            dfs(matrix,Pacificmark,0,i);
            dfs(matrix,Atlanticmark,m-1,i);
        }

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(Atlanticmark[i][j] && Pacificmark[i][j]){
                    res.push_back({i,j});
                }
            }
        }
        return res;
    }
private:
    int m,n;
    int d[4][2] = {{1,0},{-1,0},{0,1},{0,-1}};
    void dfs(vector<vector<int>>& matrix,vector<vector<bool>>& marked,int startx,int starty){
        marked[startx][starty] = true;

        for(int i=0;i<4;i++){
            int nextx = startx + d[i][0];
            int nexty = starty + d[i][1];
            if(inArea(nextx,nexty) && !marked[nextx][nexty]){
                if(matrix[nextx][nexty] >= matrix[startx][starty]){
                    dfs(matrix,marked,nextx,nexty);
                }
            }
        }
    }

    bool inArea(const int& x,const int& y){
        return x>=0 && x<m && y>=0 && y<n;
    }
};