LN : leetcode 538 Convert BST to Greater Tree

lc 538 Convert BST to Greater Tree

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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

递归 Accepted

二叉搜索树的特点是对于任一节点来说，其左子树结点都比其小，其右子树结点都比其大。所以对于本题，只需利用递归，先访问最右结点，以一个sum值记录从大到小的原数值和。

/**
* 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:
    int sum = 0;
    TreeNode* convertBST(TreeNode* root) {
        travel(root);
        return root;
    }
    void travel(TreeNode* root) {
        if (!root)  return;
        if (root->right)    travel(root->right);
        root->val = (sum += root->val);
        if (root->left)    travel(root->left);
    }
};