Leetcode114. Flatten Binary Tree to Linked List二叉树展开为链表

给定一个二叉树，原地将它展开为链表。

例如，给定二叉树

1 / 2 5 / 3 4 6

将其展开为：

1 2 3 4 5 6

class Solution {
public:
TreeNode *last = NULL;
    void flatten(TreeNode* root) 
    {
        if(root == NULL)
            return;
        TreeNode *r = root ->right;
        if(last != NULL)
        {
            last ->right = root;
        }
        last = root;
        flatten(root ->left);
        flatten(r);
        root ->left = NULL;
    }
};