算法分析與設計——LeetCode Problem.538 Convert BST to Greater Tree

題目鏈接

問題描述

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.

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

解題思路

後序遍歷BST然後把節點值逐個累加即可。

代碼如下

class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
		if (root == NULL) return NULL;
		int sum = 0;
		backorder(root, sum);
		return root;
	}
private:
	void backorder(TreeNode *r, int &sum) {
		if (r != NULL) {
			backorder(r->right, sum);
			r->val += sum;
			sum = r->val;
			backorder(r->left, sum);
		}
	}
};