题目

• 请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

思路

代码

import java.util.ArrayList;
import java.util.Stack;
/*
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) {
        ArrayList<ArrayList<Integer>> resultlist = new ArrayList<>();
        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        boolean flag = true;
        if(pRoot==null)
            return resultlist;
        stack1.push(pRoot);
        while(!stack1.isEmpty()||!stack2.isEmpty()){
            if(flag==true){
                ArrayList<Integer> templist1 = new ArrayList<>();
                while(!stack1.isEmpty()){
                TreeNode temp = stack1.pop();
                    templist1.add(temp.val);
                    if(temp.left!=null){
                        stack2.push(temp.left);
                    }
                    if(temp.right!=null){
                        stack2.push(temp.right);
                    }
                  }
                resultlist.add(templist1);
                flag=false;
            }else{
                ArrayList<Integer> templist2 = new ArrayList<>();
                while(!stack2.isEmpty()){
                TreeNode temp = stack2.pop();
                    templist2.add(temp.val);
                    if(temp.left!=null){
                        stack1.push(temp.right);
                    }
                    if(temp.right!=null){
                        stack1.push(temp.left);
                    }
                }
                resultlist.add(templist2);
                flag=true;
            }
        }
        return resultlist;
    }

}

• 牛客代码

链接：https://www.nowcoder.com/questionTerminal/91b69814117f4e8097390d107d2efbe0
来源：牛客网

 ArrayList<ArrayList<Integer> > listAll = new ArrayList<>();
        if(pRoot==null)return listAll;
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        int level = 1;
        s1.push(pRoot);
        while(!s1.isEmpty()||!s2.isEmpty()){
            ArrayList<Integer> list = new ArrayList<>();
            if(level++%2!=0){
                while(!s1.isEmpty()){
                    TreeNode node = s1.pop();
                    list.add(node.val);
                    if(node.left!=null)s2.push(node.left);
                    if(node.right!=null)s2.push(node.right);
                }
            }
            else{
                while(!s2.isEmpty()){
                    TreeNode node = s2.pop();
                    list.add(node.val);
                    if(node.right!=null)s1.push(node.right);
                    if(node.left!=null)s1.push(node.left);
                }
            }
            listAll.add(list);
        }
        return listAll;

链接：https://www.nowcoder.com/questionTerminal/91b69814117f4e8097390d107d2efbe0
来源：牛客网

ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
 
        if (pRoot == null)
        {
            return list;
        }
 
        Stack<TreeNode> stack1 = new Stack<TreeNode>();
        Stack<TreeNode> stack2 = new Stack<TreeNode>();
        stack1.push(pRoot);
        while (stack1.size() != 0 || stack2.size() != 0)
        {
            if (stack1.size() != 0)
            {
                ArrayList<Integer> list2 = new ArrayList<Integer>();
                while (stack1.size() != 0)
                {
                    TreeNode t = stack1.pop();
                    list2.add(t.val);
                    if (t.left != null)
                    {
                        stack2.push(t.left);
                    }
                    if (t.right != null)
                    {
                        stack2.push(t.right);
                    }
                }
                list.add(list2);
            } else
            {
                ArrayList<Integer> list2 = new ArrayList<Integer>();
                while (stack2.size() != 0)
                {
                    TreeNode t = stack2.pop();
                    list2.add(t.val);
                    if (t.right != null)
                    {
                        stack1.push(t.right);
                    }
                    if (t.left != null)
                    {
                        stack1.push(t.left);
                    }
 
                }
                list.add(list2);
            }
        }
        return list;