题目：

题解：

func flatten(root *TreeNode)  {
    curr := root
    for curr != nil {
        if curr.Left != nil {
            next := curr.Left
            predecessor := next
            for predecessor.Right != nil {
                predecessor = predecessor.Right
            }
            predecessor.Right = curr.Right
            curr.Left, curr.Right = nil, next
        }
        curr = curr.Right
    }
}