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

算法分析：

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）。

Java 算法实现：

/**
 * 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);
    		
    	}
    }
}