BestCoder Round #32.Negative and Positive (NP)——哈希

http://acm.split.hdu.edu.cn/showproblem.php?pid=5183

sigma（a[i],a[j])=sum[j]-sum[i-1]=K;

枚举起点： 设起点为x，分两种情况：
1.起点x为奇，a[j]-a[x-1] = K，即a[j] = a[x-1]+K。
2.起点x为偶，a[j]-(-K) = a[x-1]，即a[j] = a[x-1]-K。

由于枚举到一个起点x，需要判断a[j] (j>x) 是否出现，所以要逆序枚举。
hashmap

842 MS 29020 K 1807 B

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#define Mod 1000000007
#define lll __int64
#define ll long long
#define N 1000007
using namespace std;
const unsigned long long SMod=1000007;
ll sum[N];
struct hashmap{
    struct Edge
    {
        long long num;
        int next;
    }edges[2*N];
    int tot;
    int head[SMod+100];

    void init(){
        memset(head,-1,sizeof(head));
        tot=0;
    }

    void addedge(long long num){
        int start=num%SMod;
        edges[tot].next=head[start];
        edges[tot].num=num;
        head[start]=tot++;
    }

    int Find(long long num){
        int start=num%SMod;
        int ind;
        for(ind=head[start]; ind!=-1; ind=edges[ind].next){
            if(edges[ind].num==num)break;
        }
        return ind;
    }
}ST;


int main()
{
#ifndef ONLINE_JUDGE
    freopen("in.cpp","r",stdin);
//freopen("out.cpp","w",stdout);
#endif // ONLINE_JUDGE
    int n,i,j,cs = 1,t,x,K;
    scanf("%d",&t);
    while(t--)
    {
        ST.init();
        scanf("%d%d",&n,&K);
        sum[0] = 0;
        for(i=1;i<=n;i++) {
            scanf("%d",&x);
            if(i%2) sum[i] = sum[i-1] + x;
            else    sum[i] = sum[i-1] - x;
        }
        ST.addedge(sum[n]);
        int tag = 0;
        for(i=n-1;i>=0;i--) {
            if(i%2 == 0 && ST.Find(sum[i]+K) != -1) { tag = 1; break; }
            if(i%2 && ST.Find(sum[i]-K) != -1) { tag = 1; break; }
            ST.addedge(sum[i]);
        }
        printf("Case #%d: ",cs++);
        if(tag) puts("Yes.");
        else    puts("No.");
    }
    return 0;
}