LeetCode Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

思路：

可以先将 链表 转换成 数组，然后直接使用 数组那题的代码。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; *//** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:TreeNode *sortedListToBST(ListNode *head) {vector<int> ivec;while (head) {ivec.push_back(head->val);head = head->next;}return sortedArrayToBST(ivec);}TreeNode *sortedArrayToBST(vector<int> &num) {int n = num.size();if (n == 0)return NULL;TreeNode *head = new TreeNode(num[(n - 1) / 2]);if (n - 3 < 0)head->left = NULL;elsehead->left = creatBST(num, 0, (n - 3) / 2);head->right = creatBST(num, (n + 1) / 2, n - 1);return head;}TreeNode *creatBST(vector<int> &num, int left, int right) {if (right < left)return NULL;int mid = (left + right) / 2;TreeNode *myNode = new TreeNode(num[mid]);myNode->left = creatBST(num, left, mid - 1);myNode->right = creatBST(num, mid + 1, right);return myNode;}};

直接使用链表转化，代码更少。

class Solution {public:TreeNode *sortedListToBST(ListNode *head) {if (head == NULL)return NULL;ListNode *pslow = head, *pa = head, *pb = head->next;while (pb != NULL && pb->next != NULL) {pslow = pa;pa = pa->next;pb = pb->next->next;}TreeNode *root = new TreeNode(pa->val);pb = pa->next;pslow->next = NULL;if (pa != head)root->left = sortedListToBST(head);root->right = sortedListToBST(pb);return root;}};