题解 | #按之字形顺序打印二叉树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param pRoot TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        // write code here
        ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<>();
        if (pRoot == null) {
            return arrayLists;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(pRoot);
        boolean flag = false;
        while (!queue.isEmpty()) {
            int size = queue.size();
            ArrayList<Integer> arrayList = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                arrayList.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            if (flag) {
                Stack<Integer> stack = new Stack<>();
                for (Integer integer : arrayList) {
                    stack.add(integer);
                }
                for (int i = 0; i < arrayList.size(); i++) {
                    arrayList.set(i, stack.pop());
                }
            }
            flag = !flag;
            arrayLists.add(arrayList);
        }
        return arrayLists;
    }
}

内核还是层序遍历