Convert BST to Greater Tree问题及解法

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.

示例：

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

问题分析：

利用深度优先遍历，先遍历右子树，在遍历根节点，最后遍历左子树，对于每一个遍历到的结点root，root->val += sum(sum代表所以遍历到的节点的值的和)，sum = root->val.

过程详见代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
		int sum = 0;
		bl(root, sum);
        return root;
	}

	void bl(TreeNode* root,int& sum)
	{
		if (root == NULL) return;
		bl(root->right,sum);
		root->val += sum;
		sum = root->val;
		bl(root->left, sum);
	}
};