int sum(int n) {
    if (n == 1) return n;
    return n + sum(n - 1);
}

def sum(n):
    if n == 1:
        return n
    return n + sum(n - 1)

int decimalToBinnary(int num) {
    if (num == 0) return;
    decimalToBinnary(num / 2);
    cout << num % 2;
}

def decimal_to_binary(num):
    if num == 0:
        return
    decimal_to_binary(num // 2)
    print(num % 2, end='')

int mod(int num, int den) {
    if (num < den) return num;
    return mod(num - den, den);
}

def mod(num, den):
    if num < den:
        return num
    return mod(num - den, den)

int stringLength(string str) {
    if (str[0] == '\0') return 0;
    return 1 + stringLength(str.substr(1));
}

def string_length(str):
    if str[0] == '\0':
        return 0
    return 1 + string_length(str[1:])

void reverse(string str) {
    if (str[0] == '\0') return;
    reverse(str.substr(1));
    cout << str[0];
}

def reverse(str):
    if str[0] == '\0':
        return
    reverse(str[1:])
    print(str[0], end='')

bool isPalindrome(string str, int start, int end) {
    if (start >= end) return true;
    if (str[start] != str[end]) return false;
    return isPalindrome(str, start + 1, end - 1);
}

def is_palindrome(str, start, end):
    if start >= end:
        return True
    if str[start] != str[end]:
        return False
    return is_palindrome(str, start + 1, end - 1)

int countVowels(string str, int start, int end) {
    if (start >= end) return 0;
    int count = countVowels(str, start + 1, end);
    if (str[start] == 'a' || str[start] == 'e' || str[start] == 'i' || str[start] == 'o' || str[start] == 'u') {
        count++;
    }
    return count;
}

def count_vowels(str, start, end):
    if start >= end:
        return 0
    count = count_vowels(str, start + 1, end)
    if str[start] == 'a' or str[start] == 'e' or str[start] == 'i' or str[start] == 'o' or str[start] == 'u':
        count += 1
    return count

int sumNodes(Node* head) {
    if (head == NULL) return 0;
    return head->data + sumNodes(head->next);
}

def sum_nodes(head):
    if head == None:
        return 0
    return head.data + sum_nodes(head.next)