以下是课本提供的代码：
#include<stdio.h>
#include<stdio.h>
long int ack(int m,int n)
{
    long int value;
    if(m<0||n<0)
    {
        printf("\nThe condition of caculating is not exist,EXIT!\n");
        exit(1);
    }
    if(m==0)
    {
        value=n+1;
        printf("ack(0,%d)=%d\n",n,n+1);
    }
    else if(n==0)
    {
        value=ack(m-1,1);
        printf("ack(%d,1)=%d\n",m-1,ack(m-1,1));
    }
    else
    {
        printf("ack(%d,%d)=ack(%d,ack(%d,%d))\n",m,n,m-1,m,n-1);
        value=ack((m-1),ack(m,n-1));
    }return value;
}
int main()
{
    int mm,nn;
    long int a;
    printf("\nPLEASE ENTER M & N:");
    scanf("%d%d",&mm,&nn);
    a=ack(mm,nn);
    printf("\nack(%d,%d)=% ld\n",mm,nn,a);
    return 0;
}

PLEASE ENTER M & N:1 3
ack(1,3)=ack(0,ack(1,2))
ack(1,2)=ack(0,ack(1,1))
ack(1,1)=ack(0,ack(1,0))
ack(0,1)=2              /*这一步看得懂*/
ack(0,1)=2              /*这一步开始就看不懂了*/
ack(0,1)=2
ack(0,2)=3
ack(0,3)=4
ack(0,4)=5

ack(1,3)= 5
Press any key to continue

就是一个递归调用

同意楼上！建议好好看看递归算法，挺重要的！！