The Number of Islands problem is a popular LeetCode question (Problem #200):

Problem Statement:
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.

✅ Swift Solution Using DFS

func numIslands(_ grid: [[Character]]) -> Int {
    if grid.isEmpty { return 0 }

    var grid = grid  // make a mutable copy
    let rows = grid.count
    let cols = grid[0].count
    var islandCount = 0

    func dfs(_ r: Int, _ c: Int) {
        if r < 0 || c < 0 || r >= rows || c >= cols || grid[r][c] == "0" {
            return
        }

        grid[r][c] = "0" // mark as visited
        dfs(r + 1, c)
        dfs(r - 1, c)
        dfs(r, c + 1)
        dfs(r, c - 1)
    }

    for r in 0..<rows {
        for c in 0..<cols {
            if grid[r][c] == "1" {
                islandCount += 1
                dfs(r, c)
            }
        }
    }

    return islandCount
}

// Example usage
let inputGrid: [[Character]] = [
    ["1","1","0","0","0"],
    ["1","1","0","0","0"],
    ["0","0","1","0","0"],
    ["0","0","0","1","1"]
]

print("Number of islands: \(numIslands(inputGrid))") // Output: 3

🧠 Explanation:

Traverse the grid cell-by-cell.

When a '1' is found, it's the start of an island → increment count.

Use DFS to visit and mark all connected land as '0' (visited).

⏱️ Time Complexity:

O(m × n): Every cell is visited once.