538. Convert BST to Greater Tree

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本文介绍了一种算法，该算法将二叉搜索树（BST）转换为Greater Tree，其中每个节点的值等于原始值加上所有大于该值的节点值之和。通过逆中序遍历实现这一目标。

题目来源【Leetcode】

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.

Example:

Input: The root of a Binary Search Tree like this:
5
/ \
2 13

Output: The root of a Greater Tree like this:
18
/ \
20 13

这道题是通过中序遍历来使每一个节点加上之前比他大的那个数

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void f(TreeNode*root,int& n){
    if(root == NULL)return;
    f(root->right,n);
    root->val += n;
    n = root->val;
    f(root->left,n);
}

class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
        int n = 0;
        f(root,n);
        return root;

    }

};