Binary Tree Zigzag Level Order Traversal

Posted 2021-02-11 beiyeqingteng

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,null,null,15,7],

    3
   /   9  20
    /     15   7

 

return its zigzag level order traversal as:

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

分析：用一个flag来表示存入list的方向即可。

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11      public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
12         List<List<Integer>> allLists = new ArrayList<>();
13         if (root == null) return allLists;
14         
15         List<Integer> temp = new LinkedList<>();
16         List<TreeNode> list1 = new LinkedList<>();
17         List<TreeNode> list2 = new LinkedList<>();
18     
19         list1.add(root);
20         int direction = 1;
21     
22         while(list1.size() != 0) {
23             for (TreeNode node : list1) {
24                 if (direction == 1) {
25                     temp.add(node.val);
26                 } else {
27                     temp.add(0, node.val);
28                 }
29                 if (node.left != null) list2.add(node.left);
30                 if (node.right != null) list2.add(node.right);
31             } 
32             allLists.add(new LinkedList<Integer>(temp));
33             temp.clear();
34             list1 = list2;
35             list2 = new ArrayList<TreeNode>();
36             direction *= -1;
37         }
38         return allLists;
39     }
40 }