[LeetCode] Binary Tree Zigzag Level Order Traversal

Problem:

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]
]

Solution:O(n)

public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> ret = new ArrayList<>();
        if(root==null) 
            return ret;
        boolean order = true;
        ArrayList<TreeNode> toVisit = new ArrayList<TreeNode>();
        toVisit.add(root);
        while(!toVisit.isEmpty()){
            ArrayList<TreeNode> next = new ArrayList<TreeNode>();
            ArrayList<Integer> tmp = new ArrayList<Integer>();
            for(TreeNode node:toVisit){
                tmp.add(node.val);
            }
            ret.add(tmp);
            for(int i=toVisit.size()-1;i>=0;i--){
                TreeNode node = toVisit.get(i);
                if(order){
                    if(node.right!=null) 
                        next.add(node.right);
                    if(node.left!=null) 
                        next.add(node.left);
                }else{
                    if(node.left!=null) 
                        next.add(node.left);
                    if(node.right!=null) 
                        next.add(node.right);
                }
            }
            order = !order;
            toVisit = new ArrayList<TreeNode>(next);
        }
        return ret;
    }
}