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;
 *   }
 * }
 */
//用两个栈
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<>>
     */


    ArrayList<ArrayList<Integer>> res = new ArrayList<>();
    Stack<TreeNode> stack1 = new Stack<>();
    Stack<TreeNode> stack2 = new Stack<>();
    public void Left() {
        ArrayList<Integer> row = new ArrayList<>();
        while (!stack1.isEmpty()) {
            TreeNode node = stack1.pop();
            row.add(node.val);
            if (node.left != null) {
                stack2.add(node.left);
            }
            if (node.right != null) {
                stack2.add(node.right);
            }
        }
        res.add(row);
    }
    public void Right() {
        ArrayList<Integer> row = new ArrayList<>();
        while (!stack2.isEmpty()) {
           
            TreeNode node = stack2.pop();
            row.add(node.val);
            if (node.right != null) {
                stack1.add(node.right);
            }
            if (node.left != null) {
                stack1.add(node.left);
            }
        }
        res.add(row);
    }
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        // write code here
        if (pRoot == null) return res;
        stack1.add(pRoot);
        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            if (!stack1.isEmpty()) {
                Left();
            } else {
                Right();
            }
        }
        return res;
    }
}