Binary Tree Zigzag Level Order Traversal

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,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

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

confused what 

"{1,#,2,3}"

 means? >
read more on how binary tree is serialized on OJ.

 

<span style="font-size:14px;">public class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> ret = new ArrayList<List<Integer>>();
if (root == null)
return ret;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
int count = 0;
while(!queue.isEmpty()) {
List<Integer> list = new ArrayList<Integer>();
List<Integer> listTemp = new ArrayList<Integer>();
int size = queue.size();

for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null) {
queue.offer(node.left);
}

if (node.right != null) {
queue.offer(node.right);
}
}

if (count % 2 == 1) {
for (int i = list.size() - 1; i >= 0; i--)
listTemp.add(list.get(i));

ret.add(listTemp);
} else {
ret.add(list);
}

count++;
}

return ret;

}
}</span>