We all love recursion! Don‘t we?

Consider a three-parameter recursive function w(a, b, c):

if a <= 0 or b <= 0 or c <= 0, then w(a, b, c) returns:
1

if a > 20 or b > 20 or c > 20, then w(a, b, c) returns:
w(20, 20, 20)

if a < b and b < c, then w(a, b, c) returns:
w(a, b, c-1) + w(a, b-1, c-1) - w(a, b-1, c)

otherwise it returns:
w(a-1, b, c) + w(a-1, b-1, c) + w(a-1, b, c-1) - w(a-1, b-1, c-1)

This is an easy function to implement. The problem is, if implemented directly, for moderate values of a, b and c (for example, a = 15, b = 15, c = 15), the program takes hours to run because of the massive recursion.

The input for your program will be a series of integer triples, one per line, until the end-of-file flag of -1 -1 -1. Using the above technique, you are to calculate w(a, b, c) efficiently and print the result.

Print the value for w(a,b,c) for each triple.

1 1 1
2 2 2
10 4 6
50 50 50
-1 7 18
-1 -1 -1 

w(1, 1, 1) = 2
w(2, 2, 2) = 4
w(10, 4, 6) = 523
w(50, 50, 50) = 1048576
w(-1, 7, 18) = 1

Pacific Northwest 1999

Ignatius.L   |   We have carefully selected several similar problems for you:  1074 1078 1208 1355 1158 

#include<stdio.h>
#include<string.h>
int dp[30][30][30];
int w(int a,int b,int c)
{

	if(a<=0||b<=0||c<=0)
		return	1;
	if(a>20||b>20||c>20)
		return w(20,20,20);	
	if(dp[a][b][c])
		return dp[a][b][c];
	if(a<b&&b<c)
		return dp[a][b][c]=w(a,b,c-1)+w(a,b-1,c-1)-w(a,b-1,c);
	return dp[a][b][c]=w(a-1,b,c)+w(a-1,b-1,c)+w(a-1,b,c-1)-w(a-1,b-1,c-1);
}
int main()
{
	int a,b,c;
	while(scanf("%d%d%d",&a,&b,&c)!=EOF)
	{
		if(a==-1&&b==-1&&c==-1)
			break;
		printf("w(%d, %d, %d) = %d\n",a,b,c,w(a,b,c));
	}
}