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
对于一个静态的地图,统计岛屿个数可以使用dfs(类似于寻找一个图中的连通块个数),算法复杂度是O(m*n)。
class Solution {
int m, n;
int dr[4] = {1, 0, -1, 0};
int dc[4] = {0, -1, 0, 1};
public:
void dfs(int x, int y, vector<vector<char> >& grid) {
if (x < 0 || x >= m || y < 0 || y >= n || grid[x][y] != '1') return;
grid[x][y] = '2';
for (int i=0; i<4; i++) {
dfs(x+dr[i], y+dc[i], grid);
}
}
int numIslands(vector<vector<char> >& grid) {
int cnt = 0;
m = grid.size();
if (m == 0) return 0;
n = grid[0].size();
for (int i=0; i<m; i++) {
for (int j=0; j<n; j++) {
if (grid[i][j] == '1') {
dfs(i, j, grid);
cnt++;
}
}
}
return cnt;
}
};