文档介绍：(n)=1 n<=1时;y(n)=-y(n-2)+2*y(n-1)n>1时。编一C程序,它能对读入的任意n(n>=0且n<50),能计算并输出y(n)的值。(注:)#include""int y(n) intn;{ if(n<=1)  return1; else   return (-y(n-2)+2*y(n-1));}voidmain(){ intn; printf("pleaseinputthen:\n"); scanf("%d",&n); printf("y(%d)=%d\n",n,y(n));},它能读入一个字符串(串长<100,串中可能有空格符),计算并输出该字符串中十进制数字的个数。(注:)#include""#include""#defineMAX100voidmain(){chara[MAX];intn=0,i,j;printf("pleaseinputthechar:\n"); gets(a); i=strlen(a); if(i>=100 ||i<=0) printf("sorry!thereismorethanorlessthanonehundrednumber\n"); else if(i<100&&i>0){  for(j=0;j<i;j++)   if(a[j]>='0'&&a[j]<='9')    n++;      printf("thelengthis:%d\n",n);}}第三题编一C程序,它能读入一正整数n,(n<10),并输出如下图形:           1           222          33333           .           .        nn..... nn (共2n-1个n)           .           .          33333           222           1    ()#include""voidprint(inta,intprintnum,intcharnum,charprintchar){ inti,j; for(i=0;i<charnum;i++)  printf("%c",printchar); for(j=i;j<=i+printnum-1;j++)  printf("%d",a);  printf("\n");}voidmain(){ intn,printnum,i,charnum; charprintchar=''; printf("pleaseinputthen:\n"); scanf("%d",&n); if(n>=10||n<=0) { printf("pleaseinputnagain(n<10andn>0):\n"); scanf("%d",&n); }/*i为打印的数字printnum为打印数字的个数printchar为打印的空格charnum为打印的空格个数*/ for(i=1,printnum=2*i-1,charnum=n-i;i<=n;i++,printnum+=2,charnum--)  print(i,printnum,charnum,printchar); for(i-=2,printnum-=4,charnum=1;i>=1;i--,printnum-=2,charnum++)   print(i,printnum,charnum,printchar);},它能读入矩阵A的行数L,列数M,矩阵B的列数N,矩阵B的行数是M。L,M,N值都小于20),以及按行读入矩阵A的L*M个元素(整数)及矩阵B的M*N个元素(整数),计算并输出A与B的矩阵乘积(按行,列输出)。(输入时,相邻的两个整数用空格隔开)。()#include""#defineMAX20voidmain(){ intl,m,n,i,k,j,o,a[MAX][MAX],b[MAX][MAX],c[MAX][MAX],tmp=0; printf("pleaseinputtheL:\n"); scanf("%d",&l); printf("pleaseinputthem:\n"); scanf("%d",&m); printf("pleaseinputthen:\n"); scanf("%d",&n); if(l>=20||l<=0) { printf("pleaseinputlagain(0<l<20):\n");  scanf("%d",&l); } if(m>=20||m<