程序代码：

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<malloc.h>

int random(int max)
{
    return ((rand() << 16)|((rand() & 1) << 15)|rand()) % max;
}

int main()
{
    int n, a, c, t, g, i, j;
    double d;
    char * s, tmp;
    FILE *fo;
    
    srand(time(NULL));
    
    do{
        printf("输入序列长度：");
        scanf("%d", &n);
    }while(n <= 0);
    do{
        printf("输入A的百分占比：");
        scanf("%lf", &d);
        a = (int)(d / 100 * n + 0.5);
    }while(d < 0 || a > n);
    do{
        printf("输入C的百分占比：");
        scanf("%lf", &d);
        c = (int)(d / 100 * n + 0.5);
    }while(d < 0 || a + c > n);
    do{
        printf("输入T的百分占比：");
        scanf("%lf", &d);
        t = (int)(d / 100 * n + 0.5);
    }while(d < 0 || a + c + t > n );
    g = n - a - c - t;

    if(!(s = (char *)malloc(n + 1))) return 0;

    for(i = 0; a--; s[i++] = 'A');
    for(; c--; s[i++] = 'C');
    for(; t--; s[i++] = 'T');
    for(; g--; s[i++] = 'G');
    s[n] = '\0';

    for(i = 0; i < n; i++)
    {
        j = random(n);
        tmp = s[i];
        s[i] = s[j];
        s[j] = tmp;
    }
    
    if(!(fo = fopen("out.txt", "w")))
    {
        puts("无法创建文件out.txt\n");
        free(s);
        return 0;
    }
    
    fputs(s, fo);
    fclose(fo);
    free(s);
    return 0;
}