【ATT】Convert Sorted List to Binary Search Tree

bottom up.

    TreeNode *sortedListToBST(ListNode *head) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        //1. getLength
        if(!head)
            return NULL;
        int len = 0;
        ListNode* cur = head;
        while(cur)
        {
            len++;
            cur = cur->next;
        }
        return build(head,0,len-1);
    }
    
    TreeNode* build(ListNode*& head,int start,int end)
    {
        if(start>end)
            return NULL;
        int mid = start + (end-start)/2;
        TreeNode* leftchild = build(head,start,mid-1);
        TreeNode* parent = new TreeNode(head->val);
        head = head->next;
        parent->left = leftchild;
        parent->right = build(head,mid+1,end);
        return parent;
    }