递归将二叉树转换为双向链表

总体思路是递归，将二叉树转换为记录了头尾指针的双向链表，函数如下

void convertTreeToListHelp(TreeNode *root, TreeNode *&head, TreeNode *&tail){head = root;tail = root;if (!root)return;TreeNode *pLeftHead = NULL;    TreeNode *pLeftTail = NULL;    convertTreeToListHelp(root->left, pLeftHead, pLeftTail);    TreeNode *pRightHead = NULL;    TreeNode *pRightTail = NULL;    convertTreeToListHelp(root->right, pRightHead, pRightTail);    if (pLeftTail)    {        head = pLeftHead;        pLeftTail->right = root;        root->left= pLeftTail;    }    if (pRightHead)    {        tail = pRightTail;        root->right = pRightHead;        pRightHead->left = root;    }}

上述过程稍显罗嗦

更为简洁的方式如下：

TreeNode *head = NULL;TreeNode *tail = NULL;TreeNode convertTreeToList(TreeNode *root){If (root == NULL) return NULL;convertTreeToList(root->left);Root->left = tail;if (tail == NULL)tail = head = root;elseTail->right = root;Tail = root;convertToList(root->right);return head;}