538 convert bst to greater tree

538. 把二叉搜索树转换为累加树

go：

/**

 * Definition for a binary tree node.

 * type TreeNode struct {

 *     Val int

 *     Left *TreeNode

 *     Right *TreeNode

 * }
 */

var sum int

func convertBST(root *TreeNode) *TreeNode {
    sum = 0
    dfs(root)
    return root
}

func dfs(root *TreeNode) *TreeNode {
    if root == nil {
        return nil
    }

    root.Right = dfs(root.Right)
    sum += root.Val
    root.Val = sum
    root.Left = dfs(root.Left)

    return root
}