Leetcode#103 Binary Tree Zigzag Level Order Traversal

 1 vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
 2         vector<vector<int> > res;
 3         vector<TreeNode *> layer;
 4         bool l2r = true;
 5         
 6         layer.push_back(root);
 7         while (!layer.empty()) {
 8             vector<TreeNode *> nextLayer;
 9             vector<int> path;
10             for (auto node : layer) {
11                 if (!node)
12                     continue;
13                 path.push_back(node->val);
14                 nextLayer.push_back(node->left);
15                 nextLayer.push_back(node->right);
16             }
17             if (!l2r) {
18                 for (int i = 0; (i << 1) < path.size(); i++)
19                     swap(path[i], path[path.size() - i - 1]);
20             }
21             if (!path.empty()) 
22                 res.push_back(path);
23             layer = nextLayer;
24             l2r = !l2r;
25         }
26         
27         return res;
28 }