feat: Add solution for LeetCode problem 417

Author: WhiteHyun

pacific-atlantic-water-flow/WhiteHyun.swift

@@ -0,0 +1,60 @@
+//  
+//  417. Pacific Atlantic Water Flow
+//  https://leetcode.com/problems/pacific-atlantic-water-flow/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/29.
+//  
+
+class Solution {
+  func pacificAtlantic(_ heights: [[Int]]) -> [[Int]] {
+    var result: [[Int]] = []
+    for i in heights.indices {
+      for j in heights[i].indices where isValid(i, j, heights) {
+        result.append([i, j])
+      }
+    }
+
+    return result
+  }
+
+  private func isValid(_ i: Int, _ j: Int, _ heights: [[Int]]) -> Bool {
+    var visited: [[Bool]] = .init(
+      repeating: .init(repeating: false, count: heights[i].count),
+      count: heights.count
+    )
+    visited[i][j] = true
+    var pacificOceanFlag = i == 0 || j == 0
+    var atlanticOceanFlag = i == heights.count - 1 || j == heights[i].count - 1
+
+    func dfs(_ x: Int, _ y: Int) {
+      if pacificOceanFlag, atlanticOceanFlag { return }
+      for (dx, dy) in [(-1, 0), (1, 0), (0, -1), (0, 1)] {
+        let nx = x + dx
+        let ny = y + dy
+
+        guard heights.indices ~= nx,
+              heights[nx].indices ~= ny,
+              visited[nx][ny] == false,
+              heights[nx][ny] <= heights[x][y]
+        else {
+          continue
+        }
+
+        visited[nx][ny] = true
+        if nx == 0 || ny == 0 {
+          pacificOceanFlag = true
+        }
+        if nx == heights.endIndex - 1 || ny == heights[nx].endIndex - 1 {
+          atlanticOceanFlag = true
+        }
+
+        dfs(nx, ny)
+      }
+    }
+
+    dfs(i, j)
+
+    return pacificOceanFlag && atlanticOceanFlag
+  }
+}