[LeetCode 107] Binary Tree Level Order Traversal II

https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

http://www.lintcode.com/zh-cn/problem/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]
]

#include <list>
/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
 
 
class Solution {
    /**
     * @param root : The root of binary tree.
     * @return : buttom-up level order a list of lists of integer
     */
public:
    vector<vector<int>> levelOrderBottom(TreeNode *root) {
        // write your code here
        list<vector<int>> lstVec;
        queue<TreeNode *> q;
        
        q.push(root);
        q.push(NULL);
        while (!q.empty() && q.front() != NULL) {
            vector<int> vecSubRet;
            
            while (true) {
                TreeNode *node = q.front();
                q.pop();
                
                if (node == NULL) {
                    q.push(NULL);
                    break;
                }
                
                if (node -> left) {
                    q.push(node -> left);
                }
                
                if (node -> right) {
                    q.push(node -> right);
                }
                
                vecSubRet.push_back(node -> val);
            }
            
            lstVec.push_front(vecSubRet);
        }
        
        vector<vector<int>> ret(lstVec.begin(), lstVec.end());
        return ret;
    }
};