按之字形顺序打印二叉树

题目描述：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。

public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}

思路一：

s1存奇数层节点

s2存偶数层节点

用两个栈交替压入并弹出

import java.util.ArrayList;import java.util.Stack;public class Solution {    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        int layer = 1;        Stack<TreeNode> s1 = new Stack<>();        Stack<TreeNode> s2 = new Stack<>();        s1.add(pRoot);        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();        while (!s1.isEmpty() || !s2.isEmpty())        {            if (layer % 2 == 1)            {                ArrayList<Integer> tmp = new ArrayList<>();                while (!s1.isEmpty())                {                    TreeNode node = s1.pop();                    if (node != null)                    {                        tmp.add(node.val);                        s2.add(node.left);                        s2.add(node.right);                    }                }                if (!tmp.isEmpty())                {                    list.add(tmp);                    layer++;                }            }            else             {                ArrayList<Integer> tmp2 = new ArrayList<>();                while (!s2.isEmpty())                {                    TreeNode node2 = s2.pop();                    if (node2 != null)                    {                        tmp2.add(node2.val);                        s1.add(node2.right);                        s1.add(node2.left);                    }                }                if (!tmp2.isEmpty())                {                    list.add(tmp2);                    layer++;                }            }        }        return list;    }}

思路二：

利用Java中的LinkedList的底层实现是双向链表的特点。 

1)可用做队列，实现树的层次遍历

2)可双向遍历，奇数层时从前向后遍历，偶数层时从后向前遍历

import java.util.ArrayList;import java.util.Iterator;import java.util.LinkedList;public class Solution {    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null) return res;        boolean leftToRight = true;        LinkedList<TreeNode> queue = new LinkedList<>();        ArrayList<Integer> list = new ArrayList<>();        queue.add(null); //添加分层符        queue.add(pRoot);        while (queue.size() > 1)        {            TreeNode node = queue.poll();            if (node == null)            {                Iterator<TreeNode> iter = null;                if (leftToRight)                    iter = queue.iterator();                else                    iter = queue.descendingIterator();                leftToRight = !leftToRight;                while (iter.hasNext())                    list.add(iter.next().val);                res.add(new ArrayList<>(list));                queue.add(null);                list.clear();                continue;            }            if (node.left != null)                queue.add(node.left);            if (node.right != null)                queue.add(node.right);        }        return res;    }}

思路三：

按层序遍历分层打印的代码，添加一段判断用以倒序输出即可

import java.util.ArrayList;import java.util.LinkedList;import java.util.Queue;public class Solution {    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null) return res;        boolean leftToRight = true;        Queue<TreeNode> queue = new LinkedList<>();        ArrayList<Integer> list = new ArrayList<>();        queue.add(pRoot);        int start = 0;        int end = 1;        while (!queue.isEmpty())        {            TreeNode node = queue.poll();            list.add(node.val);            start++;            if (node.left != null)                queue.add(node.left);            if (node.right != null)                queue.add(node.right);            if (start == end)            {                start = 0;                end = queue.size();                if (leftToRight)                    res.add(list);                else                    res.add(Reverse(list));                leftToRight = !leftToRight;                list = new ArrayList<>();            }        }        return res;    }    private ArrayList<Integer> Reverse(ArrayList<Integer> list)    {        ArrayList<Integer> newList = new ArrayList<>();        for (int i = list.size() - 1; i >= 0; i--)            newList.add(list.get(i));         return newList;    }}