LeetCode-114-Flatten Binary Tree to Linked List

算法描述：

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

解题思路：从题目可以看出，用后续遍历可以解决问题。用一个全局变量存储上一个访问的点，将上一个访问的点挂在右边子树上，并将左边的子树清空。

    TreeNode* prev=nullptr;
    void flatten(TreeNode* root) {
        if(root==nullptr) return;
        flatten(root->right);
        flatten(root->left);
        root->right = prev;
        root->left = nullptr;
        prev = root;
    }