Given a string of any length which contains only digits from 0 to 9, replace each consecutive run of the digit 0 with its length.
replaceZeros("1234500362000440"); // 1234523623441
replaceZeros("123450036200044"); // 123452362344
replaceZeros("000000000000"); // 12
replaceZeros("123456789"); // 123456789