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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
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 == NULL) return;
		while(root != NULL)
		{
			if(root->left != NULL)
			{
				TreeNode *p = root->left;
				while(p->right != NULL)
				{
					p = p->right;
				}
				p->right = root->right;
				root->right = root->left;
				root->left = NULL;
			}
			root = root->right;
		}
    }
};