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

        if (pRoot == null) {
            return new ArrayList<>();
        }

        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        s1.add(pRoot);
        ArrayList<ArrayList<Integer>> array = new ArrayList();

        while (!s1.isEmpty() || !s2.isEmpty()) {

            ArrayList<Integer> list1 = new ArrayList();

            while (!s1.isEmpty()) {
                TreeNode node = s1.pop();
                list1.add(node.val);

                if (node.left != null) {
                    s2.add(node.left);
                }

                if (node.right != null) {
                    s2.add(node.right);
                }
            }

            if (list1.size() > 0) {
                array.add(list1);
            }

            ArrayList<Integer> list2 = new ArrayList();

            while (!s2.isEmpty()) {
                TreeNode node = s2.pop();
                list2.add(node.val);

                if (node.right != null) {
                    s1.add(node.right);
                }

                if (node.left != null) {
                    s1.add(node.left);
                }
            }

            if (list2.size() > 0) {
                array.add(list2);
            }

        }

        return array;
    }
}