LeetCode 538. Convert BST to Greater Tree

538. Convert BST to Greater Tree

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

題解思路：

至關於本來是升序遍歷（左根右），如今變成了降序，可是生成這些數的順序是（右根左，逐漸累加）

/**
 * 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 {
    int cur_sum = 0;
public:
    TreeNode* convertBST(TreeNode* root)
    {
        travel(root);
        return root;
    }
    
    //變成了降序，可是生成能夠用 右根左 來生成
    void travel(TreeNode *root)
    {
        if (!root) return;
        
        if (root->right)
        {
            travel(root->right);    
        }
        
        root->val = (cur_sum += root->val);
        
        if (root->left)
        {
            travel(root->left);
        }
        
    }
        
};