抽签的优化

# include <iostream>
# include <cstdio>

int main(){

    int n,m;
    int k[1000];

    scanf("%d%d",&n,&m);
    for(int i=0;i<n;i++){
        scanf("%d",&k[i]);
    }

    bool f = false;

    for(int a=0;a<n;a++)
        for(int b=0;b<n;b++)
            for(int c=0;c<n;c++)
                for(int d=0;d<n;d++)
                    if(k[a]+k[b]+k[c]+k[d]==m)
                        f = true;

    if(f)  puts("Yes\n");
    else puts("No\n");

    return 0;
}

优化1（n^3logn）：

# include <iostream>
# include <cstdio>
# include <algorithm>

using namespace std;
bool binary_search(int x);
int k[1000];
int n,m;
int main(){

    scanf("%d%d",&n,&m);
    for(int i=0;i<n;i++){
        scanf("%d",&k[i]);
    }

    sort(k,k+n);

    bool f = false;

    for(int a=0;a<n;a++)
        for(int b=0;b<n;b++)
            for(int c=0;c<n;c++)
                    if(binary_search(m-k[a]-k[b]-k[c]))
                        f = true;

    if(f)  puts("Yes\n");
    else puts("No\n");

    return 0;
} 
bool binary_search(int x){

    int left=0,right=n;

    while(right-left>=1){
        int mid = (left+right)/2;
        if(k[mid]==x)   return true;
        else if(k[mid]<x) left = mid+1;
        else right = mid;
    }

    return false;

}

优化2（n^2logn）：

# include <iostream>
# include <cstdio>
# include <algorithm>

using namespace std;
bool binary_search(int x);
int k[1000];
int kk[1000];
int n,m;
int main(){

    scanf("%d%d",&n,&m);
    for(int i=0;i<n;i++){
        scanf("%d",&k[i]);
    }

    for(int c=0;c<n;c++)//保存后两个抽取的 
        for(int d=0;d<n;d++)
            kk[c*n+d] = k[c]+k[d];



    sort(kk,kk+n*n);

    bool f = false;

    for(int a=0;a<n;a++)
        for(int b=0;b<n;b++)
            for(int c=0;c<n;c++)
                    if(binary_search(m-k[a]-k[b]))
                        f = true;

    if(f)  puts("Yes\n");
    else puts("No\n");

    return 0;
} 
bool binary_search(int x){

    int left=0,right=n;

    while(right-left>=1){
        int mid = (left+right)/2;
        if(kk[mid]==x)  return true;
        else if(kk[mid]<x) left = mid+1;
        else right = mid;
    }

    return false;

}