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

For example,

Given

1        / \       2   5      / \   \     3   4   6

The flattened tree should look like:

1    \     2      \       3        \         4          \           5            \             6

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 {public:    void flatten(TreeNode* root){        if(!root) return;        while(root){            if(root->left && root->right){                TreeNode* t = root->left;                while(t->right) t = t->right;                t->right = root->right;                root->right = NULL;            }            if(root->left){                root->right = root->left;                root->left = NULL;            }            root= root->right;        }    }};