Letter Combinations Of A Number Phone

var arr = [
    [],
    [],
    ['a', 'b', 'c'],
    ['d', 'e', 'f'],
    ['g', 'h', 'i'],
    ['j', 'k', 'l'],
    ['m', 'n', 'o'],
    ['p', 'q', 'r', 's'],
    ['t', 'u', 'v'],
    ['w', 'x', 'y', 'z']
];

var letterCombinations = function(digits) {
    if (digits.length < 1) {
        return [];
    }

    if (digits.length === 1) {
        return arr[digits[0]];
    }

    var ret = [],
        listNow = arr[digits[0]],
        listBehind = letterCombinations(digits.substring(1)),
        lenBehind = listBehind.length;

    for (var i = 0; i < listNow.length; i++) {
        for (var j = 0; j < listBehind.length; j++) {
            ret[i * lenBehind + j] = listNow[i] + listBehind[j];
        }
    }

    return ret;
};