试计算100!

在不考虑越界的情况下,可以使用递归、for循环等

方法一：

function factorial(num) {if (num < 0) {return -1;} else if (num === 0 || num === 1) {return 1;} else {return (num * factorial(num - 1));}
}
factorial(100);

方法二：

function factorial(num) {if (num < 0) {return -1;} else if (num === 0 || num === 1) {return 1;} else {for (var i = num - 1; i >= 1; i--) {num *= i;}}return num;
}
factorial(100);

• 需要考虑JS大数相乘越界问题: JS的number类型有个最大值(安全值).即2的53次方.如果超过这个值,那么JS会出现不精确的问题.

console.log(bigMut("10", "1234")); // 12340
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("");
}

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