103.binary-tree-zigzag-level-order-traversal.java

import java.util.Collections;

/*
 * @lc app=leetcode id=103 lang=java
 *
 * [103] Binary Tree Zigzag Level Order Traversal
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();

        if (root == null) {
            return result;
        }

        int level = 0;
        queue.add(root);
        
        while (!queue.isEmpty()) {
            List<Integer> curr = new ArrayList<>();
            int size = queue.size();

            for (int i = 0; i < size; i++) {
                TreeNode temp = queue.poll();
                curr.add(temp.val);

                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }

            if (level % 2 == 0) {
                result.add(curr);
            } else {
                Collections.reverse(curr);
                result.add(curr);
            }

            level++;
        }

        return result;
    }
}
// @lc code=end