给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。
例如：
给定二叉树 [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回锯齿形层次遍历如下：
[
[3],
[20,9],
[15,7]
]
代码
/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
vector<vector<int> >res;
if (!root) return res;
stack<TreeNode*> s1;
stack<TreeNode*> s2;
s1.push(root);
vector<int> out;
while (!s1.empty() || !s2.empty()) {
while (!s1.empty()) {
TreeNode *cur = s1.top();
s1.pop();
out.push_back(cur->val);
if (cur->left) s2.push(cur->left);
if (cur->right) s2.push(cur->right);
}
if (!out.empty()) res.push_back(out);
out.clear();
while (!s2.empty()) {
TreeNode *cur = s2.top();
s2.pop();
out.push_back(cur->val);
if (cur->right) s1.push(cur->right);
if (cur->left) s1.push(cur->left);
}
if (!out.empty()) res.push_back(out);
out.clear();
}
return res;
}
};
