把一個(gè)二叉樹(shù)展平成一個(gè)鏈表
借鑒了別人的思路，相當(dāng)?shù)膮柡Α?/p>

type TreeNode struct {
    Val   int
    Left  *TreeNode
    Right *TreeNode
}

func flatten(root *TreeNode) {
    node := root
    for node != nil {
        if node.Left != nil {
            tmp := node.Left
            for tmp.Right != nil {
                tmp = tmp.Right
            }
            tmp.Right = node.Right
            node.Right = node.Left
            node.Left = nil
        }
        node = node.Right
    }
}