107.Binary Tree Level Order Traversal II Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

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

vector<vector<int>> levelOrderBottom(TreeNode* root) {
    vector<vector<int>> res;
    if (!root) {
        return res;
    }
    queue<TreeNode*> t;
    t.push(root);
    while (!t.empty()) {
        vector<int> tn;
        int cnt = t.size();
        for (unsigned int i = 0; i < cnt; ++i) {
            TreeNode *cur = t.front();
            tn.push_back(cur->val);
            t.pop();
            if (cur->left)
                    t.push(cur->left);
            if (cur->right)
                t.push(cur->right);
        }
        //do not use insert() here,it cost too much time.
        //use reverse() insteadly
        res.push_back(tn);
    }
    reverse(res.begin(), res.end());
    return res;
}

REF: