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