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

大致意思是每个节点加上比这个节点大的所有值，生成一个新的数

中序遍历即可

int sum = 0;
public TreeNode convertBST(TreeNode root) {
if(root!=null)
{
convertBST(root.right);
sum+=root.val;
root.val=sum;
convertBST(root.left);
}
return root;
}