[Leetcode] 103. 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]]

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.ArrayList;import java.util.Stack;public class Solution {    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();        if(root == null) return result;        Stack<TreeNode> current = new Stack<TreeNode>();        Stack<TreeNode> next = new Stack<TreeNode>();        Stack<TreeNode> temp;        boolean normalOrder = true;        current.push(root);        while(!current.isEmpty()){            ArrayList<Integer> level = new ArrayList<Integer>();            while(!current.isEmpty()){                TreeNode node = current.pop();                level.add(node.val);                if(normalOrder){                    if(node.left != null){                        next.push(node.left);                    }                    if(node.right != null){                        next.push(node.right);                    }                } else {                    if(node.right != null){                        next.push(node.right);                    }                    if(node.left != null){                        next.push(node.left);                    }                }            }            result.add(level);            temp = current;            current = next;            next = temp;            normalOrder = !normalOrder;        }        return result;    }}