tangqiaoboy/iOS 面试题（12）：按层遍历二叉树的节点.swift Secret

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func levelOrder(_ root: TreeNode?) -> [[Int]] {
      guard let root = root else { return [] }
      
      var levelLast = root
      var last = root
      var queue: [TreeNode] = [root]
      
      var result: [[Int]] = [[Int]]()
      var levelResult: [Int] = [Int]()
      while !queue.isEmpty {
        let node = queue.removeFirst()
        levelResult.append(node.val)
        
        if let left = node.left {
          last = left
          queue.append(left)
        }
        
        if let right = node.right {
          last = right
          queue.append(right)
        }
        
        // the current level is done
        if node === levelLast {
          result.append(levelResult)
          levelResult = [Int]()
          levelLast = last
        }
      }
      
      return result
    }
}