LeetCode | Binary Tree Zigzag Level Order Traversal

题目

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

从遍历顺序上看，直接想到BFS和栈，实现时遍历顺序还是需要耐心确认的。

代码

import java.util.ArrayList;
import java.util.Stack;

public class BinaryTreeZigzagLevelOrderTraversal {
	public ArrayList
  
   > zigzagLevelOrder(TreeNode root) {
		ArrayList
   
    > results = new ArrayList
    
     >(); if (root == null) { return results; } Stack
     
       currentLevel = new Stack
      
       (); Stack
       
         nextLevel = new Stack
        
         (); boolean reverse = false; ArrayList
         
           list = new ArrayList
          
           (); currentLevel.push(root); while (!currentLevel.isEmpty()) { TreeNode node = currentLevel.pop(); list.add(node.val); if (reverse) { if (node.right != null) { nextLevel.push(node.right); } if (node.left != null) { nextLevel.push(node.left); } } else { if (node.left != null) { nextLevel.push(node.left); } if (node.right != null) { nextLevel.push(node.right); } } if (currentLevel.isEmpty()) { Stack
           
             temp = currentLevel; currentLevel = nextLevel; nextLevel = temp; results.add(new ArrayList
            
             (list)); list.clear(); reverse = !reverse; } } return results; } }