1290.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    int getDecimalValue(ListNode* head) {
        int count = 0;
        
        ListNode* temp = head;
        
        while(temp != NULL){
            count++;
            temp = temp->next;
        }
        count--;
        temp = head;
        
        int prod = pow(2, count);
        int ans = 0;
        
        while(temp != NULL){
            ans = ans + (prod * temp->val);
            temp = temp->next;
            prod = prod / 2;
        }
        return ans;
    }
};