【second】Flatten Binary Tree to Linked List

递归

    void flatten(TreeNode *root) {// Note: The Solution object is instantiated only once and is reused by each test case.flat(root);}TreeNode* flat(TreeNode* root){if(!root)return NULL;TreeNode* left_tail = flat(root->left);TreeNode* right_tail = flat(root->right);if(left_tail){left_tail->right = root->right;root->right = root->left;root->left = NULL;}if(right_tail)return right_tail;if(left_tail)return left_tail;return root;}