103. Binary Tree Zigzag Level Order Traversal -----层序遍历

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]
]


与上一题类似
不同在于，奇数层 正着存，偶数层，倒着存。

 1 class Solution {
 2     public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
 3         List<List<Integer>> res = new ArrayList<List<Integer>>();
 4         Queue<TreeNode> queue = new LinkedList<TreeNode>();
 5         if(root==null) return res;
 6         int flag =1;
 7         queue.add(root);
 8         while (!queue.isEmpty()) {
 9             int levlnum = queue.size();
10             List<Integer> res_temp = new ArrayList<Integer>();
11             for (int i = 0;i<levlnum ;i++ ){ //把每层的左右节点都保存到queue里 
12                 //并讲当层的值从queue里弹出，加到res_temp 中
13             if(queue.peek().left!=null) queue.add(queue.peek().left);
14             if(queue.peek().right!=null) queue.add(queue.peek().right);
15                 if(flag==1)
16                 res_temp.add(queue.poll().val);
17                 else
18                 res_temp.add(0,queue.poll().val);
19             }
20             res.add(res_temp);
21             flag = 1-flag;
22         }
23         return res;
24     }