洛谷P4799 [CEOI2015 Day2] 世界冰球锦标赛

题目链接：P4799 [CEOI2015 Day2] 世界冰球锦标赛 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

折半搜索：

1：先搜索左半边的票价和

2：再搜索右半边的票价和

3：找出左半边中<= m - 右半边的票价的数目 

#include 
#include 
#include 
#include 
 
using namespace std;
 
typedef long long LL;
 
const int N = 41;
 
int k;
LL n, m;
LL w[N];
int cntl, cntr;
LL lefts[1 << (N / 2)], rights[1 << (N / 2)];
 
void dfs(int u, int k, LL sum, LL res[], int &cnt)
{
    if (u == k)
    {
        res[cnt ++ ] = sum;
        return ;
    }
    
    if (sum + w[u] <= m) dfs(u + 1, k, sum + w[u], res, cnt);
    dfs(u + 1, k, sum, res, cnt);
}
 
int main()
{
    cin >> n >> m;
    for (int i = 0; i < n; i ++ ) scanf("%lld", &w[i]);
    
    k = n / 2;
    
    dfs(0, k, 0, lefts, cntl);//求左半边票价和 
    dfs(k, n, 0, rights, cntr);//求右半边票价和 
    
    sort(lefts, lefts + cntl);
    
    LL ans = 0;
    for (int i = 0; i < cntr; i ++ )//找出左半边中<= m - 右半边的票价的数目? 
    	ans += upper_bound(lefts, lefts + cntl, m - rights[i]) - lefts;
	
	cout << ans << endl;
        
        return 0;
}