103. Binary Tree Zigzag Level Order Traversal

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],

return its zigzag level order traversal as:

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

题目含义：给定一棵二叉树，从上到下，进行Z字行分层遍历的，即：本层的话从左打到右，那么下层就是从右到左，最后输出结果。

 1     public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
 2        List<List<Integer>> result = new ArrayList<>();
 3         if (root == null) return result;
 4         Queue<TreeNode> q = new LinkedList<>();
 5         q.add(root);
 6         
 7         boolean shunshizhen = false;
 8         while (!q.isEmpty())
 9         {
10             int size = q.size();
11             List<Integer> values = new ArrayList<>();
12             for (int i=0;i<size;i++)
13             {
14                 TreeNode node  = q.poll();
15                 values.add(node.val);
16                 if (shunshizhen)
17                 {
18                     if (node.left !=null) q.offer(node.left);
19                     if (node.right !=null) q.offer(node.right);      
20                 }else
21                 {
22                     if (node.right !=null) q.offer(node.right);
23                     if (node.left !=null) q.offer(node.left);
24                 }
25             }
26             shunshizhen = shunshizhen?false:true;
27             result.add(values);
28         }
29         return result;        
30     }