leetcode 102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]


class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
queue<TreeNode*> node_queue;
queue<TreeNode*> node_queue_temp;
node_queue.push(root);
vector<vector<int>> result;
vector<int> temp_result;
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) {
result.push_back(temp_result);
temp_result.clear();
queue<TreeNode*> empty;
node_queue = node_queue_temp;
swap(node_queue_temp, empty );

}
}
return result;
}
};