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Day 17 of 30 days Leetcode challenge - Number of Islands

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:

Input:
11110
11010
11000
00000

Output: 1

Example 2:

Input:
11000
11000
00100
00011

Output: 3

Solution

Use DFS to check which island each point belongs.

class Solution {
    func numIslands(_ grid: [[Character]]) -> Int {
        var grid = grid    
        var res = 0
        
        for i in 0..<grid.count {
            for j in 0..<grid[0].count {
                if grid[i][j] == "1" {
                    res += 1
                    dfs(&grid, i, j)
                }
            }
        }
        
        return res
    }
    
    func dfs(_ grid: inout [[Character]], _ row: Int, _ col: Int) {
        
        if row < 0 || col < 0 || row >= grid.count || col >= grid[0].count { return }
        
        if grid[row][col] != "1" { return }
        
        grid[row][col] = "x" //mark visited
        
        dfs(&grid, row, col + 1)
        dfs(&grid, row, col - 1)
        dfs(&grid, row, col - 1)
        dfs(&grid, row - 1, col)
        dfs(&grid, row + 1, col)
    }
}

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