大数，即超出语言所能表示的数字最大范围的数字，此时的数字只能用字符串来表示。那么如何实现两个大数相乘呢？我用JS实现了一下，代码如下：
console.log(bigMut("567", "1234")); // 699678
function bigMut(big, common) {
big += "";
common += "";
if (big.length < common.length) {
big = [common, common = big][0];
}
big = big.split("").reverse();
var oneMutManyRes = [];
var i = 0,
len = big.length;
for (; i < len; i++) {
oneMutManyRes[oneMutManyRes.length] = oneMutMany(big[i], common) + getLenZero(i);
}
var result = oneMutManyRes[0];
for (i = 1, len = oneMutManyRes.length; i < len; i++) {
result = bigNumAdd(result, oneMutManyRes[i]);
}
return result;
}
function getLenZero(len) {
len += 1;
var ary = [];
ary.length = len;
return ary.join("0");
}
function oneMutMany(one, many) {
one += "";
many += "";
if (one.length != 1) {
one = [many, many = one][0];
}
one = parseInt(one, 10);
var i = 0,
len = many.length,
resAry = [],
addTo = 0,
curItem,
curRes,
toSave;
many = many.split("").reverse();
for (; i <= len; i++) {
curItem = parseInt(many[i] || 0, 10);
curRes = curItem * one + addTo;
toSave = curRes % 10;
addTo = (curRes - curRes % 10) / 10;
resAry.unshift(toSave);
}
if (resAry[0] == 0) {
resAry.splice(0, 1);
}
return resAry.join("");
}
function bigNumAdd(big, common) {
big += "";
common += "";
var maxLen = Math.max(big.length, common.length),
bAry = big.split("").reverse(),
cAry = common.split("").reverse(),
i = 0,
addToNext = 0,
resAry = [],
fn,
sn,
sum;
for (; i <= maxLen; i++) {
fn = parseInt(bAry[i] || 0);
sn = parseInt(cAry[i] || 0);
sum = fn + sn + addToNext;
addToNext = (sum - sum % 10) / 10;
resAry.unshift(sum % 10);
}
if (resAry[0] == 0) {
resAry.splice(0, 1);
}
return resAry.join("");
}