1290. Convert Binary Number in a Linked List to Integer

class Solution {
    public int getDecimalValue(ListNode head) {
        ListNode temp = head;
        int length = 0;
        while (temp.next != null){
            temp = temp.next;
            length++;
        }
        temp = head;
        int ans = 0;
        while (temp != null && length >= 0){
            ans+=temp.val*(Math.pow(2,length));
            temp = temp.next;
            length--;
        }
        return ans;
    }
}