動機

經典的dfs

Problem

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return 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: grid = [  [1,1,1,1,0],  [1,1,0,1,0],  [1,1,0,0,0],  [0,0,0,0,0]]Output: 1

Example 2:

Input: grid = [  [1,1,0,0,0],  [1,1,0,0,0],  [0,0,1,0,0],  [0,0,0,1,1]]Output: 3

 

Constraints:

  • m == grid.length
  • n == grid[i].length
  • 1 <= m, n <= 300
  • grid[i][j] is '0' or '1'.

Sol

在做dfs時要注意

  1. 有沒有走過已經走過的解
  2. 邊界條件
def dfs(g,i,j):
    if i < 0 or i >= len(g) or j < 0 or j >= len(g[i]) or g[i][j] == '0':
        return
    else:
        g[i][j] = '0'
        dfs(g,i+1,j)
        dfs(g,i,j+1)
        dfs(g,i-1,j)
        dfs(g,i,j-1)
        
class Solution:
    def numIslands(self, g: List[List[str]]) -> int:
        ans = 0
        for i in range(0,len(g)):
            for j in range(0,len(g[i])):
                if g[i][j] == '1':
                    dfs(g,i,j)
                    ans += 1
        return ans