https://leetcode.com/problems/number-of-islands/

numberOfIslands.swift

class Solution {
    func isVisited(_ grid: [[Character]], _ i: Int, _ j: Int) -> Bool {
        return grid[i][j] == Character("2")
    }

    func markVisited(_ grid: inout [[Character]], _ i: Int, _ j: Int) {
        guard i >= 0, j >= 0, i < grid.count, j < grid[i].count, !isVisited(grid, i, j), grid[i][j] == "1" else {
            return
        }

        grid[i][j] = Character("2")

        markVisited(&grid, i-1, j)
        markVisited(&grid, i, j-1)
        markVisited(&grid, i+1, j)
        markVisited(&grid, i, j+1)
    }

    func numIslands(_ grid: [[Character]]) -> Int {
        var numIslands = 0
        var grid = grid

        for i in (0..<grid.count) {
            for j in (0..<grid[i].count) {
                guard !isVisited(grid, i, j) else {
                    continue
                }

                if grid[i][j] == "1" {
                    numIslands += 1
                    markVisited(&grid, i, j)
                }
            }
        }

        return numIslands
    }
}