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.

 

//本解法是反中序遍历所有节点，即右中左的顺序

class Solution {

    int sum = 0;

    public TreeNode convertBST(TreeNode root) {

       convert(root);

       return root;

    }

    public void convert(TreeNode node) {

       if (node == null) return;

       convert(node.right);

       node.val += sum;

       sum = node.val;

       convert(node.left);

    }

}