[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;

    }

};