LeetCode: Number of Islands

題目描述：

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

解題分析:

對於一個01矩陣A，求其中有多少片連成一片的1. 每個1可以和上下左右的1相連. 本地可以使用深度優先搜索求解，遍歷所有頂點，對每個值爲1的頂點進行DFS，訪問時把該頂點值設爲0；

代碼如下：

class Solution{
public:
 int numIslands(vector<vector<char> >& grid){
   if(grid.size()==0) return 0;
        int count=0;
        for(int i=0;i<grid.size();i++){
         for(int j=0;j<grid[0].size();j++){
          if(grid[i][j] == '1'){
           dfs(grid, i, j);
           count++;
          }
         }
        }
        return count;
    }
    void dfs(vector<vector<char> >& grid, int i, int j){
     if(i<0 || j<0 || i>=grid.size() || j>=grid[0].size() || grid[i][j]!='1') return;
     grid[i][j] = '0'; //相當於標記爲已訪問
     dfs(grid,i+1,j);
     dfs(grid,i-1,j);
     dfs(grid,i,j+1);
     dfs(grid,i,j-1);
    }
};