Leetcode：200. 岛屿的个数

题目：
给定一个由 ‘1’（陆地）和 ‘0’（水）组成的的二维网格，计算岛屿的数量。一个岛被水包围，并且它是通过水平方向或垂直方向上相邻的陆地连接而成的。你可以假设网格的四个边均被水包围。
示例 1:
输入:
11110
11010
11000
00000

输出: 1
示例 2:
输入:
11000
11000
00100
00011

输出: 3

学习DFS
代码来自英文leetcode

public int numIslands(char[][] grid) {
    int count=0;
    for(int i=0;i<grid.length;i++)
        for(int j=0;j<grid[0].length;j++){
            if(grid[i][j]=='1'){
                dfsFill(grid,i,j);
                count++;//深度搜索完一次加一次
            }
        }
    return count;
}
private void dfsFill(char[][] grid,int i, int j){
    if(i>=0 && j>=0 && i<grid.length && j<grid[0].length&&grid[i][j]=='1'){
        grid[i][j]='0';
        dfsFill(grid, i + 1, j);
        dfsFill(grid, i - 1, j);
        dfsFill(grid, i, j + 1);
        dfsFill(grid, i, j - 1);
    }
}

学习BFS
代码来自英文leetcode

public int numIslands(char[][] grid) {
    int count=0;
    for(int i=0;i<grid.length;i++)
        for(int j=0;j<grid[0].length;j++){
            if(grid[i][j]=='1'){
                bfsFill(grid,i,j);
                count++;
            }
        }
    return count;
}
private void bfsFill(char[][] grid,int x, int y){
    grid[x][y]='0';
    int n = grid.length;
    int m = grid[0].length;
    LinkedList<Integer> queue = new LinkedList<Integer>();  
    int code = x*m+y;  
    queue.offer(code);  
    while(!queue.isEmpty())  
    {  
        code = queue.poll();  
        int i = code/m;  
        int j = code%m;  
        if(i>0 && grid[i-1][j]=='1')    //search upward and mark adjacent '1's as '0'.
        {  
            queue.offer((i-1)*m+j);  
            grid[i-1][j]='0';  
        }  
        if(i<n-1 && grid[i+1][j]=='1')  //down
        {  
            queue.offer((i+1)*m+j);  
            grid[i+1][j]='0';  
        }  
        if(j>0 && grid[i][j-1]=='1')  //left
        {  
            queue.offer(i*m+j-1);  
            grid[i][j-1]='0';  
        }  
        if(j<m-1 && grid[i][j+1]=='1')  //right
        {  
            queue.offer(i*m+j+1);  
            grid[i][j+1]='0';  
        }
    } 
}

4.5回顾

class Solution {
    public int numIslands(char[][] grid) {
        int count=0;
        
        for(int i=0;i<grid.length;i++){//错误：数组~无（）；二维数组也是数组
            for(int j=0;j<grid[0].length;j++){ //错误：grid[0]未加.length
                if(grid[i][j]=='1'){
                    setzero(grid,i,j);
                    count++;
                }
            }
        }
        
        return count;
    }
    
        
    public void setzero (char[][]grid,int i,int j){

        
        grid[i][j]='0';
        if(i+1<grid.length&&grid[i+1][j]=='1') setzero(grid,i+1,j);//错误:没有加上i+1<grid.length
        
        if(i-1>=0&&grid[i-1][j]=='1') setzero(grid,i-1,j); //错误：i-1>0
        
        if(j+1<grid[0].length&&grid[i][j+1]=='1') setzero(grid,i,j+1);
        
        if(j-1>=0&&grid[i][j-1]=='1') setzero(grid,i,j-1);
    }
}