[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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > res;
        queue<TreeNode *> q;
        vector<int> ans;
        if(root==NULL) return res;
        q.push(root);
        int num=1; //一层有num个结点
        while(!q.empty())
        {
            ans.clear();
            int count=0; //记录插入了几个结点
            while(num--)
            {
                TreeNode *p=q.front();
                ans.push_back(p->val);
                q.pop();
                if(p->left) 
                {
                    q.push(p->left);
                    count++;
                }
                if(p->right) 
                {
                    q.push(p->right);
                    count++;
                }
            }
            num=count;
            res.push_back(ans);
        }
        reverse(res.begin(),res.end());
        return res;
    }
};