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]
]

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    if(root == null) return result;
    ArrayDeque<TreeNode> q = new ArrayDeque<>();
    q.offer(root);
    int level = 0;
    while(!q.isEmpty()) {
        int size = q.size();
        level++;
        LinkedList<Integer> temp = new LinkedList<>();
        boolean isReverse = level % 2 == 0;
        for(int i = 0; i < size; i++) {
            TreeNode curr = q.poll();
            if(isReverse) temp.addFirst(curr.val);
            else temp.add(curr.val);
            if(curr.left != null) q.offer(curr.left);
            if(curr.right != null) q.offer(curr.right);
        }
        result.add(temp);
    }
    return result;
}