LeetCode 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

用"中序遍历"的方式访问每个节点的值，并将该节点的值累加到一个 int sum 变量上，并用该 sum 变量更新该节点的值。要注意的是，此处的中序遍历是先遍历右子树，再访问根节点，然后再遍历左子树（因为 BST 根节点的值小于右子树所有节点的值，大于左子树所有节点的值）。

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

public void addSum(TreeNode root){
if(root!=null){
addSum(root.right);
sum+=root.val;
root.val=sum;
addSum(root.left);

}
}
}