Leetcode#107 Binary Tree Level Order Traversal II

vector<vector<int> > levelOrderBottom(TreeNode *root) {
vector<vector<int> > res;
queue<TreeNode *> layer;

layer.push(root);
while (!layer.empty()) {
queue<TreeNode *> nextLayer;
vector<int> path;

while (!layer.empty()) {
TreeNode *front = layer.front();
layer.pop();
if (!front)
continue;
path.push_back(front->val);
nextLayer.push(front->left);
nextLayer.push(front->right);
}
if (path.size() > 0)
res.push_back(path);
layer = nextLayer;
}

vector<vector<int> > tra;
for (int i = res.size() - 1; i >= 0; i--)
tra.push_back(res[i]);

return tra;
}