牛客网-剑指office-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。
**思路：**利用两个栈隔行存储二叉树的节点。一个栈从左往右依次存储一行的节点，另外一个栈从右往左依次存储下一行的节点即可。

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
	vector > Print(TreeNode* pRoot) {
		vector > ans;
		if (pRoot == nullptr) return ans;
		stack s1;
		stack s2;
		s1.push(pRoot);
		vector tmp;
		while (!s1.empty()||!s2.empty())
		{
			while (!s1.empty())
			{
				TreeNode* t = s1.top();
				s1.pop();
				if (t->left) s2.push(t->left);
				if (t->right) s2.push(t->right);
				tmp.push_back(t->val);
			}
			if(!tmp.empty()) ans.push_back(tmp);
			tmp.clear();
			while (!s2.empty())
			{
				TreeNode* t = s2.top();
				s2.pop();
				if (t->right) s1.push(t->right);
				if (t->left) s1.push(t->left);
				tmp.push_back(t->val);
			}
			if (!tmp.empty()) ans.push_back(tmp);
			tmp.clear();
		}
		return ans;
	}

};