牛客 剑指offer：二叉树的之字形遍历

题目：二叉树的之字形遍历

思路：借助队列。

队列实现层序遍历，然后反转奇数行。

代码：

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder (TreeNode root) {
        // write code here
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        queue.offer(root);
        while (! queue.isEmpty()) {
            int n = queue.size();
            ArrayList<Integer> array = new ArrayList<>();
            for (int i = 0; i < n; i ++) {
                TreeNode temp = queue.poll();
                array.add(temp.val);
                if (temp.left != null) {
                    queue.offer(temp.left);
                }
                if (temp.right != null) {
                    queue.offer(temp.right);
                }
            }
            res.add(array);
        }
        for (int i = 1; i < res.size(); i += 2) {
            Collections.reverse(res.get(i));
        }
        return res;
    }
}