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).

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:

Binary tree level order traversal with a reverse operation on every even row.

Code:

public class Solution {

    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        boolean zigzag = false;
        while (!q.isEmpty()) {
            int size = q.size();
            ArrayList<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                list.add(node.val);
                if (node.left != null) {
                    q.offer(node.left);
                }
                if (node.right != null) {
                    q.offer(node.right);
                }
            }
            if (zigzag) {
                Collections.reverse(list);
            }
            zigzag = !zigzag;
            res.add(list);
        }
        return res;
    }
}