# 按之字形顺序打印二叉树

## 题目：请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，即第一行按照从左到右的顺序打印，第二层按照从右到左顺序打印，第三行再按照从左到右的顺序打印，其他以此类推。

### 结点定义

private static class BinaryTreeNode {
private int val;
private BinaryTreeNode left;
private BinaryTreeNode right;
public BinaryTreeNode() {
}
public BinaryTreeNode(int val) {
this.val = val;
}
@Override
public String toString() {
return val + "";
}
}

### 代码实现

import java.util.LinkedList;
import java.util.List;
public class Test61 {
private static class BinaryTreeNode {
private int val;
private BinaryTreeNode left;
private BinaryTreeNode right;
public BinaryTreeNode() {
}
public BinaryTreeNode(int val) {
this.val = val;
}
@Override
public String toString() {
return val + "";
}
}
public static void print(BinaryTreeNode root) {
if (root == null) {
return;
}
int flag = 0;
BinaryTreeNode node;
while (current.size() > 0) {
// 从最后一个开始取
node = current.remove(current.size() - 1);
System.out.printf("%-3d", node.val);
// 当前是从左往右打印的，那就按从左往右入栈
if (flag == 0) {
if (node.left != null) {
}
if (node.right != null) {
}
}
// 当前是从右往左打印的，那就按从右往左入栈
else {
if (node.right != null) {
}
if (node.left != null) {
}
}
if (current.size() == 0) {
flag = 1 - flag;
List<BinaryTreeNode> tmp = current;
current = reverse;
reverse = tmp;
System.out.println();
}
}
}
public static void main(String[] args) {
BinaryTreeNode n1 = new BinaryTreeNode(1);
BinaryTreeNode n2 = new BinaryTreeNode(2);
BinaryTreeNode n3 = new BinaryTreeNode(3);
BinaryTreeNode n4 = new BinaryTreeNode(4);
BinaryTreeNode n5 = new BinaryTreeNode(5);
BinaryTreeNode n6 = new BinaryTreeNode(6);
BinaryTreeNode n7 = new BinaryTreeNode(7);
BinaryTreeNode n8 = new BinaryTreeNode(8);
BinaryTreeNode n9 = new BinaryTreeNode(9);
n1.left = n2;
n1.right = n3;
n2.left = n4;
n2.right = n5;
n3.left = n6;
n3.right = n7;
n4.left = n8;
n4.right = n9;
print(n1);
}
}