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。
分析
用一个布尔量控制从左到右还是从右到左的顺序；如果为真则他的左右孩子按照从左到右的顺序放入list中，否则按照从右到左的顺序，当本行结束之后，再把list中的数据放入一个result中。其中用一个null值作为层次的分隔符。
代码
1 package com.leetcode.printTreeNode;
2
3 import java.util.ArrayList;
4 import java.util.Iterator;
5 import java.util.LinkedList;
6
7 /**
8 * Created by Feng on 2016/11/8.
9 * 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，
10 * 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。
11 */
12 public class PrintTreeNode {
13 public static void main(String[] args) {
14 TreeNode node1 = new TreeNode(1);
15 TreeNode node2 = new TreeNode(2);
16 TreeNode node3 = new TreeNode(3);
17 TreeNode node4 = new TreeNode(4);
18 TreeNode node5 = new TreeNode(5);
19 TreeNode node6 = new TreeNode(6);
20 TreeNode node7 = new TreeNode(7);
21 TreeNode node8 = new TreeNode(8);
22 TreeNode node9 = new TreeNode(9);
23 node1.left = node2;
24 node1.right = node3;
25 node2.left = node4;
26 node2.right = node5;
27 node3.left = node6;
28 node3.right = node7;
29 node4.left = node8;
30 node4.right = node9;
31 ArrayList<ArrayList<Integer>> result = printTreeNode(node1);
32 for (ArrayList list : result) {
33 for (int i = 0; i < list.size(); i++) {
34 System.out.print(list.get(i) + " ");
35 }
36 }
37 }
38
39 public static ArrayList<ArrayList<Integer>> printTreeNode(TreeNode pRoot) {
40
41 ArrayList<ArrayList<Integer>> result = new ArrayList<>();
42 if (pRoot == null) {
43 return result;
44 }
45
46 ArrayList<Integer> list = new ArrayList<>();
47 LinkedList<TreeNode> queue = new LinkedList<>();
48 queue.addLast(null);//层分隔符
49 queue.addLast(pRoot);
50 boolean leftToRight = true;
51 while (queue.size() != 1) {
52 TreeNode node = queue.removeFirst();
53 if (node == null) {//到达层分隔符
54 Iterator<TreeNode> iter = null;
55 if (leftToRight) {
56 iter = queue.iterator();//从前往后遍历
57 } else {
58 iter = queue.descendingIterator();//从后往前遍历
59 }
60 leftToRight = !leftToRight;
61 while (iter.hasNext()) {
62 TreeNode temp = (TreeNode) iter.next();
63 list.add(temp.val);
64 }
65 result.add(new ArrayList<Integer>(list));
66 list.clear();
67 queue.addLast(null);//添加层分隔符
68 continue;//一定要continue
69 }
70 if (node.left != null) {
71 queue.addLast(node.left);
72 }
73 if (node.right != null) {
74 queue.addLast(node.right);
75 }
76 }
77
78 return result;
79 }
80
81 }
82
83 class TreeNode {
84 int val = 0;
85 TreeNode left = null;
86 TreeNode right = null;
87
88 public TreeNode(int val) {
89 this.val = val;
90 }
91 }
运行结果
1 3 2 4 5 6 7 9 8
Process finished with exit code 0