Leetcode143: 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.

与排序好的数组转化为二分搜索树的题相似，可以先把链表转化为数组在转化为树。

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* 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:
TreeNode* createTree(vector<int> &a, int left, int right)
{
if(left>right)
return NULL;
int mid = (left+right)/2;
TreeNode *node = new TreeNode(a[mid]);
TreeNode *Tleft = createTree(a, left, mid-1);
TreeNode *Tright = createTree(a, mid+1, right);
node->left = Tleft;
node->right = Tright;
return node;
}
TreeNode* sortedListToBST(ListNode* head) {
vector<int> a;
ListNode *ptr = head;
while(ptr)
{
a.push_back(ptr->val);
ptr = ptr->next;
}
return createTree(a, 0, a.size()-1);
}
};