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leetcode笔记：200. number of islands

admin 11月 14, 2020 0

#	Title	Difficulty	Topic
200	Number of Islands	Medium	Depth-frist Search; Breadth-first Search; Union Find

Description

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:

Input:
11110
11010
11000
00000

Output: 1

Example 2:

Input:
11000
11000
00100
00011

Output: 3

DFS

每遍历到一个字符1，就把该岛上的1全部变为0。

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

时间复杂度为$$O((mn)^2)$$。