SpiralOrderTraverse，螺旋遍历二叉树，利用两个栈

问题描述：s型遍历二叉树，或者反s型遍历二叉树

算法分析：层序遍历二叉树只需要一个队列，因为每一层都是从左往右遍历，而s型遍历二叉树就要用两个栈了，因为每次方向相反。

public static void SpiralOrderTraverse(TreeNode t) {
		Stack<TreeNode> stack1 = new Stack<>();
		Stack<TreeNode> stack2 = new Stack<>();
		stack1.push(t);
		while (!stack1.isEmpty() || !stack2.isEmpty()) {
			while (!stack1.isEmpty()) {
				TreeNode tn = stack1.pop();
				System.out.print(tn.val + " ");
				if (tn.right != null) {
					stack2.push(tn.right);
				}
				if (tn.left != null) {
					stack2.push(tn.left);
				}
			}
			while (!stack2.isEmpty()) {
				TreeNode tn = stack2.pop();
				System.out.print(tn.val + " ");
				if (tn.left != null) {
					stack1.push(tn.left);
				}
				if (tn.right != null) {
					stack1.push(tn.right);
				}
			}
		}
	}