feat: Add solution for LeetCode problem 200

Author: WhiteHyun

number-of-islands/WhiteHyun.swift

@@ -0,0 +1,48 @@
+//  
+//  200. Number of Islands
+//  https://leetcode.com/problems/number-of-islands/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/28.
+//  
+
+class Solution {
+  func numIslands(_ grid: [[Character]]) -> Int {
+    var visited: [[Bool]] = .init(
+      repeating: .init(repeating: false, count: grid[0].count),
+      count: grid.count
+    )
+    var islands = 0
+
+    for row in 0 ..< grid.count {
+      for col in 0 ..< grid[row].count where grid[row][col] == "1" && !visited[row][col] {
+        visited[row][col] = true
+        dfs(grid, &visited, row, col)
+        islands += 1
+      }
+    }
+
+    return islands
+  }
+
+  private func dfs(
+    _ grid: [[Character]],
+    _ visited: inout [[Bool]],
+    _ x: Int,
+    _ y: Int
+  ) {
+    for (dx, dy) in [(-1, 0), (1, 0), (0, -1), (0, 1)] {
+      let nx = dx + x
+      let ny = dy + y
+      guard grid.indices ~= nx,
+            grid[x].indices ~= ny,
+            visited[nx][ny] == false,
+            grid[nx][ny] == "1"
+      else {
+        continue
+      }
+      visited[nx][ny] = true
+      dfs(grid, &visited, nx, ny)
+    }
+  }
+}