二叉树展开成链表

 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class  {
public:
    TreeNode* last;
    void flatten(TreeNode* root) {
        if (root == NULL) return;
        flatten(root->right);
        flatten(root->left);
        root->right = last;
        root->left = NULL;
        last = root;
    }
};