Binary Tree Zigzag Level Order Traversal.java

/*
    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).

    Link: https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

    Example: 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: None

    Source: None
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();

        if (root == null) {
            return result;
        }

        Stack<TreeNode> currLevel = new Stack<TreeNode>();
        Stack<TreeNode> nextLevel = new Stack<TreeNode>();
        Stack<TreeNode> tmp;
        
        currLevel.push(root);
        boolean normalOrder = true;

        while (!currLevel.isEmpty()) {
            List<Integer> currLevelResult = new ArrayList<Integer>();

            while (!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);

                if (normalOrder) {
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                } else {
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                }
            }

            result.add(currLevelResult);
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }

        return result;
    }
}