#include <stdio.h>
int main()
{
    int n, i, sum = 0;

    printf("输入一个正整数: ");
    scanf("%d",&n);

    for(i=1; i <= n; ++i)
    {
        sum += i;   // sum = sum+i;
    }

    printf("Sum = %d",sum);

    return 0;
}

#include <stdio.h>
int main()
{
    int n, i, sum = 0;

    printf("输入一个正整数: ");
    scanf("%d",&n);

    i = 1;
    while ( i <=n )
    {
        sum += i;
        ++i;
    }

    printf("Sum = %d",sum);

    return 0;
}

输入一个正整数: 100
Sum = 5050

#include <stdio.h>
int addNumbers(int n);

int main()
{
    int num;
    printf("输入一个整数: ");
    scanf("%d", &num);
    printf("Sum = %d",addNumbers(num));
    return 0;
}

int addNumbers(int n)
{
    if(n != 0)
        return n + addNumbers(n-1);
    else
        return n;
}