POJ 题目1579 Function Run Fun(记忆搜索)
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Function Run Fun
Time Limit: 1000MSMemory Limit: 10000KTotal Submissions: 14815Accepted: 7659
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.
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 12 2 210 4 650 50 50-1 7 18-1 -1 -1
Sample Output
w(1, 1, 1) = 2w(2, 2, 2) = 4w(10, 4, 6) = 523w(50, 50, 50) = 1048576w(-1, 7, 18) = 1
经典记忆化搜索:就是将每次搜索的结果保存在一个数组内,计算过的结果不必再重复计算;
#include <stdio.h>#include <string.h>int vis[22][22][22];//用来保存每次递归调用的结果int w(int a,int b,int c){if (a<=0 || b <= 0 || c<=0) return 1;if (a>20 || b > 20 || c>20) returnvis[20][20][20] = w(20, 20, 20);//直接返回vis[20][20][20]里面的值 if(vis[a][b][c]) return vis[a][b][c];//如果已经计算过,直接返回if (a<b && b < c) returnvis[a][b][c] = w(a,b,c-1) + w(a,b-1,c-1) - w(a,b-1,c) ; //每次返回前保存结果在vis[][][];else return vis[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 ans,a,b,c;while(scanf("%d%d%d",&a,&b,&c),!(a==-1&&b==-1&& c==-1)){memset(vis,0,sizeof(vis));printf("w(%d, %d, %d) = %d\n",a,b,c,w(a,b,c));}return 0;}
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