[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.

这题的关键是能找出当前链表的中间节点，然后再递归左右的子链表，开始的时候程序先计算链表总厂，然后传入两个前后索引指针，最后每次递归找出中间节点即可。

/**
 * 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:
    int calLen(ListNode *node)
    {
        int len = 0;
        while(node)
        {
            len++;
            node = node->next;
        }
        return len;
    }
    
    TreeNode *createTree(ListNode *node, int left, int right)
    {
        if (left > right)
            return NULL;
            
        int mid = (left + right) / 2;
        
        ListNode *p = node;
        
        for(int i = left; i < mid; i++)
            p = p->next;
            
        TreeNode *leftNode = createTree(node, left, mid - 1);
        TreeNode *rightNode = createTree(p->next, mid + 1, right);
        
        TreeNode *tNode = new TreeNode(p->val);
        
        tNode->left = leftNode;
        tNode->right = rightNode;
        
        return tNode;        
    }
    
    TreeNode *sortedListToBST(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len = calLen(head);
        return createTree(head, 0, len - 1);
    }
};