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一、

4! = 1 * 2 * 3 * 4
5! = 1 * 2 * 3 * 4 * 5

#include<stdio.h>#pragma warning:(disable:4996)int main(){   	int i = 0;	int ret = 1;	int n = 0;	scanf("%d", &n);	for(i=1; i<=n; i++)	{   		ret *= n;//累计相乘	}	printf("%d\n", ret);	return 0;}

二、

1! + 2! +3! + …

//思路：//1		 +//1*2    +//1*2*3  +//... ...#define _CRT_SECURE_NO_WARNINGS#include<stdio.h>int main(){   	int i = 0;	int j = 0;	int ret = 1;	int sum = 0;	int n = 0;	scanf("%d", &n);	for(i=1; i<=n; i++)	{   		ret = 1;//在求下一个阶乘时重新赋值,避免把上一阶乘的值与下一阶乘的值重复相乘		for(j=1; j<=i; j++)		{   			ret *= j;			sum += ret;		}	}	printf("%d\n", sum);	return 0;}

上面这种写法时间复杂度高，效率低。优化如下：

//思路：//1*1  = 1!//1!*2 = 2!//2!*3 = 3!//... ...#define _CRT_SECURE_NO_WARNINGS#include<stdio.h>int main(){   	int i = 0;	int ret = 1;	int sum = 0;	int n = 0;	scanf("%d", &n);	for(i=1;i<=n;i++)	{   		ret *= i;		sum += ret;	}	return 0;}