Golang written test question: Write a function that receives an integer parameter n and outputs the factorial result of n

Today, the AI ​​written test question tool we developed, ai Bianshi - the AI ​​programmer written test system, gave me an intermediate Golang question, which is this question: " Please write a function that receives an integer parameter n and outputs the factorial result of n" , I hope I can write a function that outputs the factorial result of n. I didn't think there was any problem at first, so I probably wrote an implementation as follows:

// 循环计算n的阶乘
func factorial(n int) int {
	var result = 1
	for i := 1; i <= n; i++ {
		result *= i
	}
	return result
}

Or you can use recursion:

// 计算n的阶乘
func factorial1(n int) int {
	if n == 1 {
		return 1
	}
	return n * factorial1(n-1)
}

Later I felt something was wrong. It didn’t look like an intermediate question.

I ran a simple test example and found that these two implementations actually overflowed int at 21.

21的阶乘是-4249290049419214848
21的阶乘是-4249290049419214848

Oh, sure enough there is a hidden test point.

Golang actually provides us with a large number library:

math.Big

The loop implementation version of the Big library is as follows:

// 计算n的阶乘，使用math/big包
func factorial3(n int) *big.Int {
	var result = big.NewInt(1)
	for i := 1; i <= n; i++ {
		result.Mul(result, big.NewInt(int64(i)))
	}
	return result
}

or recursive version

// 计算n的阶乘，使用math/big包,递归实现
func factorial8(n int) *big.Int {
	if n == 1 {
		return big.NewInt(1)
	}
	var result = big.NewInt(int64(n))
	return result.Mul(result, factorial8(n-1))
}

Finally, I chose to submit the loop version. The recursive version has unnecessary consumption when the recursion depth is deep, so looping is fine.