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
        TreeNode head = pRoot;
        ArrayList<ArrayList<Integer>> res = new ArrayList();
        if(head == null) return res;
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        Stack<TreeNode> s2 = new Stack<TreeNode>();
        s1.push(head);
        while(!s1.isEmpty()||!s2.isEmpty()){
            ArrayList<Integer> temp = new ArrayList<Integer>();
            while(!s1.isEmpty()){
                TreeNode node = s1.pop();
                temp.add(node.val);
                if(node.left!= null){
                    s2.push(node.left);
                }
                if(node.right != null){
                    s2.push(node.right);
                }
            }
            if(temp.size()!= 0) res.add(new ArrayList<Integer>(temp));
            temp.clear();
            while(!s2.isEmpty()){
                TreeNode node = s2.pop();
                temp.add(node.val);
                if(node.right!= null){
                    s1.push(node.right);
                }
                if(node.left!=null){
                    s1.push(node.left);
                }
            }
            if(temp.size()!= 0) res.add(new ArrayList<Integer>(temp));
            temp.clear();
        }
        

        return res;
    }
}

与之前的层序遍历类似 只是需要使用两个栈来倒换方向