【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.

 

class Solution {
public:
    int listlen(ListNode * node)
    {
        int len=0;
        while(node)
        {
            ++len;
            node=node->next;
        }
        return len;
    }

    TreeNode * createBST(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=createBST(node,left,mid-1);
        TreeNode *rightNode=createBST(p->next,mid+1,right);

        TreeNode *root=new TreeNode(p->val);
        root->left=leftNode;
        root->right=rightNode;

        return root;
    }

    TreeNode* sortedListToBST(ListNode* head) {
        int len=listlen(head);
        return createBST(head,0,len-1);
    }
};