114. Flatten Binary Tree to Linked List

/*Given a binary tree, flatten it to a linked list in-place.For example,Given         1        / \       2   5      / \   \     3   4   6The flattened tree should look like:   1    \     2      \       3        \         4          \           5            \             6click to show hints.Hints:If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.*//** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {private:    TreeNode* pre=NULL;public:    void flatten(TreeNode* root) {        if(!root) return;        flatten(root->right);        flatten(root->left);        root->left=NULL;        root->right=pre;        pre=root;    }};