Golang Leetcode 538. Convert BST to Greater Tree.go

var sum int

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

func helper(root *TreeNode) {
	if root == nil {
		return
	}
	helper(root.Right)

	root.Val += sum
	sum = root.Val
	helper(root.Left)
}