LeetCode200. Number of Islands 基礎dfs

題目：https://leetcode.com/problems/number-of-islands/description/

dfs尋找聯通區域個數，使用遞歸會超時，要用棧模擬遞歸，代碼如下：

class Solution {
public:
    
    void dfs(vector<vector<char>> grid,vector<vector<int>> &visited ,int i,int j){
        visited[i][j] = 1;
		stack<pair<int,int>>s;
		s.push(make_pair(i,j));
		while(!s.empty()){
			pair<int,int> p = s.top();
			s.pop();
			i=p.first;
			j = p.second;
			visited[p.first][p.second] = 1;
			if(i+1<grid.size()&&grid[i+1][j] == '1'&&visited[i+1][j]==0)
				s.push(make_pair(i+1,j));
			if(j+1<grid[0].size()&&grid[i][j+1] == '1'&&visited[i][j+1]==0)
				s.push(make_pair(i,j+1));
			if(i-1>=0&&grid[i-1][j]=='1'&&visited[i-1][j]==0)
				s.push(make_pair(i-1,j));
			if(j-1>=0&&grid[i][j-1] == '1'&&visited[i][j-1]==0)
				s.push(make_pair(i,j-1));
		}
    }
    
    int numIslands(vector<vector<char>>& grid) {
        if(grid.size()==0) return 0;
        int m = grid.size();
        int n = grid[0].size();
        int res = 0;
        
        vector<vector<int>> visited(m,vector<int>(n,0));
        for(int i = 0;i<m;++i){
            for(int j=0;j<n;++j){
                if(visited[i][j]==0&&grid[i][j]=='1')
                {
                    res++;
                    dfs(grid,visited,i,j);
                }
            }
        }
        return res;
    }
};