一、题目内容

二、题解

解法1：

思路

临时数组使用队列，正序尾插，逆序头插入

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return new ArrayList<List<Integer>>();
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        queue.offer(root);
        int level = 0;
        while (!queue.isEmpty()) {
            LinkedList<Integer> temp = new LinkedList<Integer>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode head = queue.poll();
                if (level % 2 == 0) {
                    temp.add(head.val);
                } else {
                    temp.addFirst(head.val);
                }
                if (head.left != null) {
                    queue.offer(head.left);
                }
                if (head.right != null) {
                    queue.offer(head.right);
                }
            }
            result.add(temp);
            level++;
        }
        return result;
    }
}