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

这道题仔细想想其实是inorder traversal，只不过不同的是顺序变成了右中左，然后记录之前所有node的总和。

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

        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p = root;
        while(p != null) {
            stack.push(p);
            p = p.right;
        }

        int last = 0;

        while(stack.size() > 0) {
            TreeNode temp = stack.pop();
            temp.val = temp.val + last;
            last = temp.val;

            System.out.println(last);

            if(temp.left != null) {
                p = temp.left;
                while(p != null) {
                    stack.push(p);
                    p = p.right;
                }
            }
        }

        return root;
    }
}

或者也可以递归

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

        convertBST(root.right);
        sum += root.val;
        root.val = sum;
        convertBST(root.left);

        return root;
    }
}