103. Binary Tree Zigzag Level Order Traversal

 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         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
 7         queue.offer(root);
 8         int lastLevelCnt = 1;
 9         int curLevelCnt = 0;
10         List<Integer> curLevel = new ArrayList<Integer>();
11         boolean reverse = false;
12         while(!queue.isEmpty()) {
13             TreeNode cur = queue.poll();
14             curLevel.add(cur.val);
15             lastLevelCnt--;
16             if(cur.left != null) {
17                 queue.offer(cur.left);
18                 curLevelCnt++;
19             }
20             if(cur.right != null) {
21                 queue.offer(cur.right);
22                 curLevelCnt++;
23             }
24             if(lastLevelCnt == 0) {
25                 if(reverse) {
26                     Collections.reverse(curLevel);
27                 }
28                 reverse = !reverse;
29                 res.add(curLevel);
30                 lastLevelCnt = curLevelCnt;
31                 curLevelCnt = 0;
32                 curLevel = new ArrayList<Integer>();
33             }
34         }
35         return res;
36     }