按之字形顺序打印二叉树
请实现一个函数按照之字顺序打印二叉树，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，第三行再按照从左到右的顺序打印，以此类推
分析：
使用两个栈实现
public class PrintTreeInZigzag<Key extends Comparable<Key>> {
private class Node {
public Key key;
public Node left, right;
public Node(Key key) {
this.key = key;
}
}
public void print(Node node) {
if (node == null) {
return;
}
Stack<Node>[] levels = new Stack[2];
int current = 0;
int next = 1;
levels[current].push(node);
while (!levels[0].empty() || !levels[1].empty()) {
Node top = levels[current].pop();
System.out.printf("%d\t", top.key);
if (current == 0) {
if (top.left != null) levels[next].push(top.left);
if (top.right != null) levels[next].push(top.right);
} else {
if (top.right != null) levels[next].push(top.right);
if (top.left != null) levels[next].push(top.left);
}
if (levels[current].empty()) {
System.out.println();
current = 1 - current;
next = 1 - next;
}
}
}
}
