200. Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

Solution:

DFS.

Iterate the grid and if grid[i][j] =='1', we increment the count and start from i, j as entrance and DFS the grid, mark all connected '1' to something else (marked it as visited). We will find each island in one round of DFS, Because we mark them as visited as we DFS the grid, we will be assured that we are not counting duplicate '1'. Ideally, we should restore all the '1' after we get the count. But I am lazy, so I didn't do it in my solution. 

 public int numIslands(char[][] grid) {  
    if(grid==null || grid.length==0 || grid[0].length==0) return 0;  
     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'){  
           count++;  
           visitLand(grid,i,j);  
         }  
       }  
     }  
     return count;  
   }  
   public void visitLand(char[][] grid,int i,int j){  
     if(i<0|| i>=grid.length) return;  
     if(j<0|| j>=grid[0].length) return;  
     if(grid[i][j]!='1') return;  
     grid[i][j]='2';  
     visitLand(grid,i-1,j);  
     visitLand(grid,i+1,j);  
     visitLand(grid,i,j-1);  
     visitLand(grid,i,j+1);  
   }