103.Binary Tree Zigzag Level Order Traversal

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The main idea of ​​the title: The binary tree is printed in a z-shaped manner, that is, the odd-numbered lines are printed normally, and the even-numbered lines are printed in reverse order. Examples are as follows:

Method 1: Use the layer order traversal of 102, and then use Collections.reverse(list) to flip the list when there are even rows. The code is as follows (takes 2ms):

 1     public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
 2         List<List<Integer>> res = new ArrayList<List<Integer>>();
 3         if(root == null) {
 4             return res;
 5         }
 6         Queue<TreeNode> q = new LinkedList<TreeNode>();
 7         q.offer(root);
 8         int level = 0;
 9         while(!q.isEmpty()) {
10             level++;
11             int cnt = q.size();
12             List<Integer> listLevel = new ArrayList<Integer>();
13             for(int i = 0; i < cnt; i++) {
14                 TreeNode tmp = q.poll();
15                 listLevel.add(tmp.val);
16                 if(tmp.left != null) {
17                     q.offer(tmp.left);
18                 }
19                 if(tmp.right != null) {
20                     q.offer(tmp.right);
21                 }
22             }
23             //对list进行翻转
24             if(level % 2 == 0)
25                 Collections.reverse(listLevel);
26             res.add(new ArrayList<Integer>(listLevel));
27         }
28         return res;
29     }

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