递推递归练习第c题解析

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#include<iostream>
#include<cmath>
using namespace std;
int i,j,k,d[21][21][21];

void f (int d[21][21][21])
{
for (i=0;i<21;i++)
for (j=0;j<21;j++)
for (k=0;k<21;k++)
{if (i==0||j==0||k==0) d[i][j][k]=1;
else if (i<j&&j<k) d[i][j][k]=d[i][j][k-1]+d[i][j-1][k-1]-d[i][j-1][k];
else  d[i][j][k]=d[i-1][j][k]+d[i-1][j-1][k]+d[i-1][j][k-1]-d[i-1][j-1][k-1];

}
}
int main()
{
f(d);
int  n,a,b,c;
while (cin>>a>>b>>c)
{  if (a<=0||b<=0||c<=0) cout<<d[0][0][0]<<endl;
else if (a>20||b>20||c>20) cout<<d[20][20][20]<<endl;
else  cout<<d[a][b][c]<<endl;
}
}