POJ 1579 Function Run Fun 【记忆化搜索入门】

题目传送门：http://poj.org/problem?id=1579

Function Run Fun

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 20560		Accepted: 10325

Description

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.

Input

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.

Output

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

Sample Input

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

Sample Output

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

Source

Pacific Northwest 1999

题意概括：

要求写一个函数 w( a, b, c) 处理输入数据（多测试）；

①如果 a < 0 || b < 0 || c < 0；直接返回w( a, b, c )；

②如果 a > 20 || b > 20 || c > 20；返回w( 20, 20, 20 )；

③如果 a < b && b < c ；返回 w(a, b, c-1) + w(a, b-1, c-1) - w(a, b-1, c)；

④ 其他情况返回w(a-1, b, c) + w(a-1, b-1, c) + w(a-1, b, c-1) - w(a-1, b-1, c-1) ；

解题思路：

记忆化搜索裸题。

AC code：

 /// POJ 1579 记忆化搜索入门
 #include <cstdio>
 #include <iostream>
 #include <algorithm>
 #include <cstring>
 #include <cmath>
 #define ll long long int
 #define INF 0x3f3f3f3f
 using namespace std;

 const int MAXN = ;
 int d[MAXN][MAXN][MAXN];

 int dfs(int a, int b, int c)
 {
     if(a <=  || b <=  || c <= ) return ;
     if(a >  || b >  || c > ) return dfs(, , );
     if(d[a][b][c]) return d[a][b][c];
     if(a < b && b < c) d[a][b][c] = dfs(a, b, c-)+dfs(a, b-, c-) - dfs(a, b-, c);
     else d[a][b][c] = dfs(a-, b, c)+dfs(a-, b-, c)+dfs(a-, b, c-)-dfs(a-,b-,c-);
     return d[a][b][c];
 }
 int main()
 {
     int ans, A, B, C;
     memset(d, , sizeof(d));
     while(~scanf("%d%d%d", &A, &B, &C))
     {
         if(A == - && B == - && C == -) break;
         ans = dfs(A, B, C);
         printf("w(%d, %d, %d) = %d\n", A, B, C, ans);
     }
     return ;
 }