417. Pacific Atlantic Water Flow

最新推荐文章于 2025-07-29 14:24:32 发布

转载最新推荐文章于 2025-07-29 14:24:32 发布 · 78 阅读

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原文链接：http://www.cnblogs.com/tsunami-lj/p/6516918.html

本文介绍了一种算法，用于找到一个矩阵中可以流向太平洋和大西洋的单元格坐标。利用深度优先搜索策略，从矩阵边界开始遍历，标记能够到达两侧的单元格。最终返回同时可达两海洋的所有单元格坐标。

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:

The order of returned grid coordinates does not matter.
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).
解题思路：pacific是左侧和上侧Atlantic是右侧和下侧。用两个数组分别记录从pacific出发从低往高能到的点和从Atlantic出发能到的点。两者交集即为能到两边的点。

int dx[]={1,0,-1,0};
int dy[]={0,-1,0,1};
class Solution {
public:
    void dfs(int n,int m,vector<vector<int>>& matrix,vector<vector<int>> &vis,int height,int x,int y){
        if(x<0||y<0||x>=n||y>=m||vis[x][y]||matrix[x][y]<height)return;
        vis[x][y]=1;
        for(int k=0;k<4;k++){
            int xx=x+dx[k];
            int yy=y+dy[k];
            dfs(n,m,matrix,vis,matrix[x][y],xx,yy);
        }
    }
    vector<pair<int, int>> pacificAtlantic(vector<vector<int>>& matrix) {
        if(matrix.empty())return {};
        int n=matrix.size(),m=matrix[0].size();
        vector<pair<int, int>>res;
        vector<vector<int>> pacific(n,vector<int>(m,0));
        vector<vector<int> >atlantic(n,vector<int>(m,0));
        for(int i=0;i<n;i++){
            dfs(n,m,matrix,pacific,0,i,0);
            dfs(n,m,matrix,atlantic,0,i,m-1);
        }
        for(int i=0;i<m;i++){
            dfs(n,m,matrix,pacific,0,0,i);
            dfs(n,m,matrix,atlantic,0,n-1,i);
        }
       for(int i=0;i<n;i++){
           for(int j=0;j<m;j++){
               if(pacific[i][j]&&atlantic[i][j]){
                   res.push_back({i,j});
               }
           }
       }
       return res;
    }
};

转载于:https://www.cnblogs.com/tsunami-lj/p/6516918.html