An introduction to recursive functions

Code to find the sum of first n natural numbers:
def sum(n,s=0):
    if n==0:
        return s;
    else:
        s=s+n;
        return sum(n-1,s)

#main
n=int(input("Enter a number:"))
s=sum(n)
print(s)

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Code to find the largest digit in an integer:
def largest(n,l=0):
    if n==0:
        return l;
    else:
        if n%10>l:
            l=n%10
        return largest(n//10,l)

#main
n=int(input("Enter an integer:"))
p=largest(n)
print("Largest Digit",p)
 

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Code to find the HCF between 2 numbers:
def HCF(n,d):
    if n%d==0:
        return d
    else:
        r=n%d
        n=d
        d=r
        return HCF(n,d)

#main
n=int(input("Enter a number:"))
m=int(input("Enter another number:"))
h=HCF(n,m)
print("HCF=",h)