Binary Tree Level Order Traversal

    3
   /   9  20
    /     15   7

[
  [3],
  [9,20],
  [15,7]
]

 1 class Solution {
 2 public:
 3     vector<vector<int>> levelOrder( TreeNode *root ) {
 4         vector<vector<int>> levels;
 5         if( !root ) { return levels; }
 6         vector<TreeNode*> nodes( 1, root );
 7         while( !nodes.empty() ) {
 8             vector<TreeNode*> new_nodes;
 9             vector<int> values;
10             for( auto iter = nodes.begin(); iter != nodes.end(); ++iter ) {
11                 values.push_back( (*iter)->val );
12                 if( (*iter)->left ) { new_nodes.push_back( (*iter)->left ); }
13                 if( (*iter)->right ) { new_nodes.push_back( (*iter)->right ); }
14             }
15             levels.push_back( values );
16             nodes = new_nodes;
17         }
18         return levels;
19     }
20 };