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]
]

Anaylysis

Similar to Binary Tree Level Order Traversal, only difference is to make odd and even level different.
Use BFS.

Code

public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root==null) {return result;}
        Queue<TreeNode> queue = new LinkedList<TreeNode>();

        queue.add(root);
        int level = 0;
        while (!queue.isEmpty()){
            int size = queue.size();
            List<Integer> subList = new ArrayList<Integer>();
            for (int i=0; i<size;i++){
                if(queue.peek().left!=null) {queue.add(queue.peek().left);}
                if(queue.peek().right!=null) {queue.add(queue.peek().right);}
                if(level%2==0) {subList.add(queue.poll().val);}
                else {subList.add(0, queue.poll().val);}
            }
            result.add(subList);
            level++;
        }

        return result;
    }
}