Question

How can I write a function that takes a string denoting a natural number (>0) such as “100100000000” or “1234567890123456788912345678912345678901234567890” and returns a string denoting the input number decreased by 1? I cannot convert this string to an integer because it could overflow.

I am open to implementing this function in any popular language. I personally know c, C++, Java, javascript, python, and php.

Answer 1

k=len(x)-1

while(True):
    if x[k]!='0':
        x[k]-=1
        break
    else:
        x[k]='9'
        k--

I am leaving boundary conditions for you to work out.