[Leetcode] 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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

Analysis:

class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > ret;
        if (root == NULL)
            return ret;
        queue<TreeNode*> q;
        int currLevel = 1;
        int nextLevel = 0;
        q.push(root);
        vector<int> row;
        while(!q.empty())
        {
            TreeNode* t = q.front();
            q.pop();
            currLevel--;
            row.push_back(t->val);
            if (t->left != NULL)
            {
                q.push(t->left);
                nextLevel++;
            }
            if (t->right != NULL)
            {
                q.push(t->right);
                nextLevel++;
            }                
            
            if (currLevel == 0)
            {
                currLevel = nextLevel;
                nextLevel = 0;
                ret.insert(ret.begin(), row);
                row.clear();
            }
        }
        
        return ret;
    }
};