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Binary Tree Zigzag Level Order Traversal

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Description

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

Solution

Two things need paying attention:

Using stack plus direction flag to traverse each node.
Pay attention how to instantiate a nested list
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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ret = new LinkedList<>();
        if (root == null) return ret;
        
        boolean direction = false;
        LinkedList<TreeNode> stack = new LinkedList<>();
        
        
        //Initialization
        List<Integer> firstLine = new LinkedList<>();
        firstLine.add(root.val);
        ret.add(firstLine);
        stack.add(root);
        
        while(stack.isEmpty() == false) {
            LinkedList<TreeNode> tempStack = new LinkedList<TreeNode>(stack);
            stack = new LinkedList<TreeNode>(); 
            List<Integer> localRet = new LinkedList<Integer>();
            while(tempStack.isEmpty() == false) {
                TreeNode tempNode = tempStack.removeFirst();
                if(direction == true) {
                    if(tempNode.left != null) { stack.addFirst(tempNode.left); localRet.add(tempNode.left.val); }
                    if(tempNode.right != null) { stack.addFirst(tempNode.right); localRet.add(tempNode.right.val); }
                }else{
                    if(tempNode.right != null) { stack.addFirst(tempNode.right); localRet.add(tempNode.right.val); }
                    if(tempNode.left != null) { stack.addFirst(tempNode.left); localRet.add(tempNode.left.val); }
                }
            }
            if(localRet.isEmpty() == false) ret.add(localRet);
            direction = !direction;
        }
        
        return ret;
    }
    
}