feat: Add solution for LeetCode problem 261

Author: WhiteHyun

graph-valid-tree/WhiteHyun.swift

@@ -0,0 +1,42 @@
+//  
+//  261. Graph Valid Tree
+//  https://leetcode.com/problems/graph-valid-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/07/06.
+//  
+
+final class Solution {
+  func validTree(_ n: Int, _ edges: [[Int]]) -> Bool {
+    guard edges.count == n - 1
+    else {
+      return false
+    }
+
+    var dictionary: [Int: [Int]] = [:]
+    var visited: Set<Int> = []
+
+    for edge in edges {
+      dictionary[edge[0], default: []].append(edge[1])
+      dictionary[edge[1], default: []].append(edge[0])
+    }
+
+    func dfs(parent: Int, node: Int) {
+      if visited.contains(node) {
+        return
+      }
+
+      visited.insert(node)
+
+      if let childNodes = dictionary[node] {
+        for childNode in childNodes where childNode != parent {
+          dfs(parent: node, node: childNode)
+        }
+      }
+    }
+
+    dfs(parent: -1, node: 0)
+
+    return visited.count == n
+  }
+}