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,null,null,15,7],

3

/ \ 9 20 / \ 15 7

return its zigzag level order traversal as:

[ [3], [20,9], [15,7] ]

Solution

/**
 * 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<TreeNode> currentLevel = new ArrayList<>();
        if (root != null) {
            currentLevel.add(root);
        }

        boolean left = false;
        while (!currentLevel.isEmpty()) {
            List<Integer> cur = currentLevel.stream().map(item -> item.val).collect(Collectors.toList());
            result.add(cur);

            List<TreeNode> nextLevel = new ArrayList<>();


            if (left) {

                for(int i = currentLevel.size() - 1; i >= 0; i --) {
                    TreeNode curNode = currentLevel.get(i);
                    if (curNode.left != null) {
                        nextLevel.add(curNode.left);
                    }

                    if (curNode.right != null) {
                        nextLevel.add(curNode.right);
                    }
                }

                left = false;


            } else {

                for(int i = currentLevel.size() - 1; i >= 0; i --) {
                    TreeNode curNode = currentLevel.get(i);

                    if (curNode.right != null) {
                        nextLevel.add(curNode.right);
                    }

                    if (curNode.left != null) {
                        nextLevel.add(curNode.left);
                    }


                }

                left = true;
            }

            currentLevel = nextLevel;
        }

        return result;
    }
}