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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<List<Integer>> res;
    
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        res = new ArrayList<>();
        if (root == null) return res;
        
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        boolean flag = false;
        while (!q.isEmpty()) {
            int size = q.size();
            LinkedList<Integer> temp = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                if (flag)
                    temp.addLast(node.val);
                else
                    temp.addFirst(node.val);
                if (node.right != null)
                    q.add(node.right);
                if (node.left != null)
                    q.add(node.left);
            }
            flag = !flag;
            res.add(temp);
        }
        return res;
    }
}