HDU 1427 速算24点

http://acm.hdu.edu.cn/showproblem.php?pid=1427

简单的dfs枚举所有情况就行

符号的组合实际上有两种

((ab)c)d 和 (ab)(cb)

 dfs(sum+pos, cur+1, num[cur+1]);
  	  dfs(sum-pos, cur+1, num[cur+1]);
 	   dfs(sum*pos, cur+1, num[cur+1]);
 	   if(pos != 0 && sum%pos == 0)
   	     dfs(sum/pos, cur+1, num[cur+1]);
	dfs(sum, cur+1, pos+num[cur+1]);
  	  dfs(sum, cur+1, pos-num[cur+1]);
	    dfs(sum, cur+1, pos*num[cur+1]);
    if(num[cur+1] != 0 && pos%num[cur+1] == 0)
        dfs(sum, cur+1, pos/num[cur+1]);

这一段是核心,枚举了所有的组合

#include <iostream>
#include <fstream>
#include <cstring>
#include <algorithm>
using namespace std;
int num[4];
bool flag = 0;
void dfs(int sum,  int cur, int pos)
{
    if(flag)
        return;
    if(cur == 3)
    {
        if(sum + pos == 24)
            flag = 1;
        if(sum - pos == 24)
            flag = 1;
        if(sum * pos == 24)
            flag = 1;
        if(pos != 0 && sum%pos == 0 && sum/pos == 24)
            flag = 1;
        return;
    }
   
	 dfs(sum+pos, cur+1, num[cur+1]);
  	  dfs(sum-pos, cur+1, num[cur+1]);
 	   dfs(sum*pos, cur+1, num[cur+1]);
 	   if(pos != 0 && sum%pos == 0)
   	     dfs(sum/pos, cur+1, num[cur+1]);
	dfs(sum, cur+1, pos+num[cur+1]);
  	  dfs(sum, cur+1, pos-num[cur+1]);
	    dfs(sum, cur+1, pos*num[cur+1]);
    if(num[cur+1] != 0 && pos%num[cur+1] == 0)
        dfs(sum, cur+1, pos/num[cur+1]);
}int main(void){//  freopen("1.txt", "r", stdin);    char str[100];    while(cin>>str)    {        if(str[0] == 'A')            num[0] = 1;        else if(str[0] == 'J')            num[0] = 11;        else if(str[0] == 'Q')            num[0] = 12;        else if(str[0] == 'K')            num[0] = 13;        else if(str[0] == '1' && str[1] == '0')            num[0] = 10;        else            num[0] = str[0] - '0';        for(int i = 1; i < 4; i ++)        {            cin>>str;            if(str[0] == 'A')                num[i] = 1;            else if(str[0] == 'J')                num[i] = 11;            else if(str[0] == 'Q')                num[i] = 12;            else if(str[0] == 'K')                num[i] = 13;            else if(str[0] == '1' && str[1] == '0')                num[i] = 10;            else                num[i] = str[0] - '0';        }        sort(num, num+4);        flag = 0;        do        {            dfs(num[0], 1, num[1]);        }        while(next_permutation(num, num+4)&&!flag);        if(flag)            cout<<"Yes"<<endl;        else            cout<<"No"<<endl;    }    return 0;}