[Leetcode]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

class Solution {
public:
/*algorithm: BFS
1)if grid[i][j] is '1', count++,BFS search from (i,j),mark them to '#'
2)scan grid row by row
time O(m*n), space O(1)
*/
void bfs(vector<vector<char>>& grid,int x,int y,int m,int n){
if(x < 0 || x >= m ||y < 0 || y >= n)return;
if(grid[x][y] == '1'){
grid[x][y] = '#';
bfs(grid,x-1,y,m,n);//top
bfs(grid,x,y+1,m,n);//right
bfs(grid,x+1,y,m,n);//down
bfs(grid,x,y-1,m,n);//left
}
}
int numIslands(vector<vector<char>>& grid) {
int m = grid.size();
if(m < 1)return 0;
int n = grid[0].size();
if(n < 1)return 0;
int cnt = 0;
for(int r = 0;r < m;r++){
for(int c = 0;c < n;c++){
if(grid[r][c] == '1'){
++cnt;
bfs(grid,r,c,m,n);
}
}
}
return cnt;
}
};