[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],

]

 解题思路：

充分利用数据结构。

/**

 * 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) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        vector<vector<int>> ans;

        if(root == NULL) return ans;

        vector<TreeNode*> cur;

        vector<int> curVal;

        cur.push_back(root);

        curVal.push_back(root -> val);

        while(!cur.empty())

        {

            vector<vector<int>>::iterator it = ans.begin();

            ans.insert(it, curVal);

            vector<TreeNode*> tmp;

            vector<int> tmpVal;

            for(int i = 0;i < cur.size();i++)

            {

                if(cur[i] -> left != NULL) 

                {

                    tmp.push_back(cur[i] -> left);

                    tmpVal.push_back(cur[i] -> left -> val);

                }

                if(cur[i] -> right != NULL) 

                {

                    tmp.push_back(cur[i] -> right);

                    tmpVal.push_back(cur[i] -> right -> val);

                }

            }

            cur = tmp;

            curVal = tmpVal;

        }

        return ans;

    }

};