feat: Add solution for LeetCode problem 102

Author: WhiteHyun

binary-tree-level-order-traversal/WhiteHyun.swift

@@ -0,0 +1,107 @@
+//  
+//  102. Binary Tree Level Order Traversal
+//  https://leetcode.com/problems/binary-tree-level-order-traversal/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/12.
+//  
+
+// MARK: - Node
+
+private class Node<T> {
+  let element: T
+  var next: Node<T>?
+
+  init(element: T, next: Node<T>? = nil) {
+    self.element = element
+    self.next = next
+  }
+}
+
+// MARK: - Queue
+
+final class Queue<T> {
+  private var head: Node<T>?
+  private var tail: Node<T>?
+  private var count: Int = 0
+
+  var isEmpty: Bool {
+    count == 0
+  }
+
+  func enqueue(_ element: T) {
+    let newNode = Node(element: element)
+
+    if let tailNode = tail {
+      tailNode.next = newNode
+    } else {
+      head = newNode
+    }
+
+    tail = newNode
+    count += 1
+  }
+
+  func dequeue() -> T? {
+    guard let headNode = head
+    else {
+      return nil
+    }
+
+    let element = headNode.element
+    head = headNode.next
+
+    if head == nil {
+      tail = nil
+    }
+
+    count -= 1
+    return element
+  }
+
+  func peek() -> T? {
+    head?.element
+  }
+
+  func clear() {
+    head = nil
+    tail = nil
+    count = 0
+  }
+}
+
+// MARK: ExpressibleByArrayLiteral
+
+extension Queue: ExpressibleByArrayLiteral {
+  convenience init(arrayLiteral elements: T...) {
+    self.init()
+    for element in elements {
+      enqueue(element)
+    }
+  }
+}
+
+class Solution {
+  func levelOrder(_ root: TreeNode?) -> [[Int]] {
+    guard let root else { return [] }
+    var array: [[Int]] = []
+    let queue: Queue<(node: TreeNode, layer: Int)> = [(root, 0)]
+
+    while let (node, layer) = queue.dequeue() {
+      // array index 범위에 layer가 들어있지 않으면, 마지막 요소에 빈 배열 추가
+      if (array.indices ~= layer) == false {
+        array.append([])
+      }
+      array[layer].append(node.val)
+
+      if node.left != nil {
+        queue.enqueue((node.left!, layer + 1))
+      }
+      if node.right != nil {
+        queue.enqueue((node.right!, layer + 1))
+      }
+    }
+
+    return array
+  }
+}