leetcode 102. Binary Tree Level Order Traversal

 1 vector<vector<int> > levelOrder(TreeNode *root) 
 2     {
 3         vector<vector<int> > result;
 4         vector<int> line;
 5         queue<TreeNode *> qtree;
 6         TreeNode *visit = root;
 7         int current_level_size = 1, next_level_size = 0;
 8         
 9         if (visit != NULL)
10             qtree.push(visit);
11         while (!qtree.empty())
12         {
13             visit = qtree.front();
14             qtree.pop();
15             current_level_size--;
16             line.push_back(visit->val);
17             
18             if (visit->left != NULL)
19             {
20                 qtree.push(visit->left);
21                 next_level_size++;
22             }
23             if (visit->right != NULL)
24             {
25                 qtree.push(visit->right);
26                 next_level_size++;
27             }
28             if (current_level_size == 0) // all this level nodes are visited.
29             {
30                 result.push_back(line);
31                 line.clear();
32                 current_level_size = next_level_size;
33                 next_level_size = 0;
34             }
35         }
36 
37         return result;
38     }