Ackermann function
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The Ackermann function is a classic recursive example in computer science. It is a function that grows very quickly (in its value and in the size of its call tree). It is defined as follows:
n+1 if m=0 A(m, n) = A(m-1, 1) if m>0 and n=0 A(m-1, A(m,n-1)) if m>0 and n>0
Its arguments are never negative and it always terminates. Write a function which returns the value of A(m, n). Arbitrary precision is preferred (since the funciton grows so quickly), but not required.
Java
<java>public static BigInteger ack(BigInteger m, BigInteger n){ if(m.equals(BigInteger.ZERO)) return n.add(BigInteger.ONE);
if(m.compareTo(BigInteger.ZERO) > 0 && n.equals(BigInteger.ZERO)) return ack(m.subtract(BigInteger.ONE), BigInteger.ONE);
if(m.compareTo(BigInteger.ZERO) > 0 && n.compareTo(BigInteger.ZERO) > 0) return ack(m.subtract(BigInteger.ONE), ack(m, n.subtract(BigInteger.ONE)));
return null; }</java>