Convert BST to Greater Tree

问题描述：

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

样例：

Given a binary search Tree ｀{5,2,3}｀:

              5
            /   \
           2     13

Return the root of new tree

             18
            /   \
          20     13

解题思路：

二叉排序树按中序遍历的先右后左得到的数组是从大到小的排列的，正好变换之后就是每一元素加上前面的元素的和。用了两次得中序遍历，第一次得到数组，处理后在重新更改二叉树。

代码：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root the root of binary tree
     * @return the new root
     */
     vector<int>tree;
   void InorderTraversal(TreeNode *root) {
        if (root== NULL)return ;
        InorderTraversal(root->right);
       tree.push_back(root->val);
        InorderTraversal(root->left);
    }
    int k=0;
    void InorderTraversal1(TreeNode *root) {
        if (root== NULL)return ;
        InorderTraversal1(root->right);
      root->val=tree[k];k++;
        InorderTraversal1(root->left);
    }
    TreeNode* convertBST(TreeNode* root) {
        // Write your code here
        InorderTraversal(root);
        for(int i=1;i<tree.size();i++){
            tree[i]=tree[i]+tree[i-1];
        }
        InorderTraversal1(root);
        return root;
    }
    
};

感想：

方法虽然很简单但是时间效率太低了 需要两次遍历才能得到结果。