Integer to Roman numeral

Convert an integer num (1 <= num <= 3999) to its Roman numeral string. Roman numerals use I=1, V=5, X=10, L=50, C=100, D=500, M=1000, with subtractive pairs IV=4, IX=9, XL=40, XC=90, CD=400, CM=900. For example 1994 = MCMXCIV.