题目要求：
Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]
/**
* Definition for a binary tree node.
* 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) {
queue<TreeNode*> node_queue;
queue<TreeNode*> node_queue_temp;
node_queue.push(root);
vector<vector<int>> result;
vector<int> temp_result;
bool direction = true;
while(!node_queue.empty()) {
TreeNode* now_node = node_queue.front();
node_queue.pop();
if(now_node != NULL) {
temp_result.push_back(now_node->val);
node_queue_temp.push(now_node->left);
node_queue_temp.push(now_node->right);
}
if(node_queue.empty() && temp_result.size() > 0) {
if(direction)
result.push_back(temp_result);
else {
reverse(temp_result.begin(), temp_result.end());
result.push_back(temp_result);
}
direction = !direction;
temp_result.clear();
queue<TreeNode*> empty;
node_queue = node_queue_temp;
swap(node_queue_temp, empty );
}
}
return result;
}
};