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

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：

 1 /// POJ 1579 记忆化搜索入门
 2 #include <cstdio>
 3 #include <iostream>
 4 #include <algorithm>
 5 #include <cstring>
 6 #include <cmath>
 7 #define ll long long int
 8 #define INF 0x3f3f3f3f
 9 using namespace std;
10
11 const int MAXN = 23;
12 int d[MAXN][MAXN][MAXN];
13
14 int dfs(int a, int b, int c)
15 {
16     if(a <= 0 || b <= 0 || c <= 0) return 1;
17     if(a > 20 || b > 20 || c > 20) return dfs(20, 20, 20);
18     if(d[a][b][c]) return d[a][b][c];
19     if(a < b && b < c) d[a][b][c] = dfs(a, b, c-1)+dfs(a, b-1, c-1) - dfs(a, b-1, c);
20     else d[a][b][c] = dfs(a-1, b, c)+dfs(a-1, b-1, c)+dfs(a-1, b, c-1)-dfs(a-1,b-1,c-1);
21     return d[a][b][c];
22 }
23 int main()
24 {
25     int ans, A, B, C;
26     memset(d, 0, sizeof(d));
27     while(~scanf("%d%d%d", &A, &B, &C))
28     {
29         if(A == -1 && B == -1 && C == -1) break;
30         ans = dfs(A, B, C);
31         printf("w(%d, %d, %d) = %d\n", A, B, C, ans);
32     }
33     return 0;
34 }

原文地址：https://www.cnblogs.com/ymzjj/p/9495426.html