200_NumberofIslands

---

title: 200. Number of Islands tags:
- backtracking - bfs - Union Find categories: leetcode comments: false

problem

solution

option 1 - backtracking

將拜訪過的位置，原地修改其陣列的值為 0

class Solution {
public:
    void dfs(vector<vector<char>> & grid,int i, int j){
        int n = grid.size(), m = grid[0].size();
        if(i<0 || j<0 || i>n-1 || j>m-1 || grid[i][j] =='0') return;
        
        grid[i][j] = '0';
        dfs(grid, i-1,j);
        dfs(grid, i+1,j);
        dfs(grid, i,j-1);
        dfs(grid, i, j+1);
        
    }
    int numIslands(vector<vector<char>>& grid) {
        
        int n = grid.size(), m = grid[0].size();
        int count = 0;
        for(int i=0;i<n;++i){
            for(int j = 0;j<m;++j){
                if(grid[i][j] == '1'){
                    dfs(grid, i, j);
                    count++;
                }
            }
        }
        return count;
        
    }
};

option 2 bfs

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        
        int n = grid.size(), m = grid[0].size(),count= 0;
        queue<vector<int>> q;
        for(int i=0;i<n;++i){
            for(int j = 0 ; j<m ;++j){
                
                if(grid[i][j] == '1'){
                    count++;
                    q.push({i,j});
                    while(!q.empty()){
                        vector<int> cur = q.front();
                        q.pop();
                        int x = cur[0], y = cur[1];
                        if(x< 0 || y<0 || x>n-1 || y>m-1 || grid[x][y] == '0') continue;
                        grid[x][y] = '0';
                        push({x+1,y});
                        q.push({x-1,y});
                        q.push({x,y+1});
                        q.push({x,y-1});
                    }
                }
                
            }
        }
        return count;
    }
};

option 3 - *Union Find

analysis

• option 1
  • time complexity O(n*m)
  • space complexity O(n*m)
• option 2
  • time complexity O(n*m)
  • space complexity O(n*m)