Python version of code, it can be easily changed into java
def addKGroupSumUntilSumLengthSmallerThanK(inputstr, k): curstr = inputstr while len(curstr) > k: group = len(curstr) // k if len(curstr) % k != 0: group += 1 itrvals = ['']*group //char[] itrvals = new char[group]; for g in range(group): subsum = 0 for i in range(k): if i +g*k == len(curstr): break subsum += int(curstr[i+g*k]) itrvals[g] = str(subsum) curstr = ''.join(itrvals) //curstr = String.valueof(itrvals) return curstr