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 {
    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        if(pRoot == null) return new ArrayList<>();
        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        TreeNode node;
        stack1.push(pRoot);
        while(!stack1.isEmpty() || !stack2.isEmpty()){
            ArrayList<Integer> list1 = new ArrayList<>();
            while(!stack1.isEmpty()){
                node = stack1.pop();
                list1.add(node.val);
                if(node.left != null) stack2.push(node.left);
                if(node.right != null) stack2.push(node.right);
            }
            if(!list1.isEmpty()) res.add(list1);
            ArrayList<Integer> list2 = new ArrayList<>();
            while(!stack2.isEmpty()){
                node = stack2.pop();
                list2.add(node.val);
                if(node.right != null) stack1.push(node.right);
                if(node.left != null) stack1.push(node.left);
            }
            if(!list2.isEmpty()) res.add(list2);
        }
        return res;
    }

}