POJ1840 离散化+二分 OR hash_链接法 （简单题）

1 题意

2 分析

Note:代码一的二分，如果用STL的lower_bound()，速度很慢、甚至超时；而二分自己写的话速度就很快。

代码一：离散化+二分 （1000ms+ / 5000ms）

代码二：hash （500ms+ / 5000ms）

3 

代码一：

#include <iostream>#include <stdio.h>#include <algorithm>#include <string.h>#define LI(x) x*x*xusing namespace std;//-(a1x1+a2x2)=(a3x3+a4x4+a5x5)const int maxn=10010;int res_left[maxn];int number[5];int cur;void Ini(){    cur=0;    for(int i=-50;i<=50;i++){        if(i==0)    continue;        for(int j=-50;j<=50;j++){            if(j==0)    continue;            res_left[cur++]=-1*(number[0]*LI(i)+number[1]*LI(j));        }    }    //cout<<"cur:  "<<cur<<endl;    sort(res_left,res_left+cur);}int Search_binary(int sum){    int left=0,right=cur-1,mid=0,res=0;    while(left<=right){        mid=(left+right)>>1;        if(res_left[mid]<sum){            left=mid+1;///!!!   Wrong：left=mid        }        else if(sum<res_left[mid]){            right=mid-1;///!!!  Wrong:  right=mid        }        else if(sum==res_left[mid]){            res=1;            int mid_left=mid-1;            int mid_right=mid+1;            while(sum==res_left[mid_left]&&mid_left>=0){                res++;                mid_left--;            }            while(sum==res_left[mid_right]&&mid_right<=cur-1){                res++;                mid_right++;            }            break;        }    }    return res;}/*int Search_binary(int sum){    int pos=lower_bound(res_left,res_left+cur,sum)-res_left;    int res=0;    if(pos<cur&&res_left[pos]==sum)     res=1;    else    res=0;    for(int i=pos+1;i<cur;i++){        if(res_left[i]==sum){            res++;        }    }    return res;}*/int main(){    //freopen("out.txt","w",stdout);    while(~scanf("%d",&number[0])){        for(int i=1;i<5;i++)   scanf("%d",&number[i]);        Ini();        int sum=0,res=0;        for(int i=-50;i<=50;i++){            if(i==0)    continue;            for(int j=-50;j<=50;j++){                if(j==0)    continue;                for(int k=-50;k<=50;k++){                    if(k==0)    continue;                    sum=LI(i)*number[2]+LI(j)*number[3]+LI(k)*number[4];                    res+=Search_binary(sum);                }            }        }        cout<<res<<endl;    }    return 0;}

代码二：

#include <iostream>#include <stdio.h>#include <string.h>#include <algorithm>#define LF(x) x*x*xusing namespace std;const int maxn=1000007; //maxn为散列表hash_[]的长度。 maxn < int数组长度(262144*10)，const int MOD=maxn; // MOD除了作为MOD以外，还是hash_head[]的长度。MOD < KEY_max:  18750000,KEY_min:   -18750000，< int值（+/- 2147483647）int cur=0;//当前指针int hash_head[MOD];//Hash(KEY) - 对应链表的首指针struct Node{    int key;    int next;}hash_[maxn];//下标由cur赋值，逐渐递增int a[5];void Insert_hash(int key){    int temp=(key>0)?key:-key;//因为数组下标不会有负数    int cor_num=(temp)%MOD;    hash_[cur].key=key;    hash_[cur].next=hash_head[cor_num];    hash_head[cor_num]=cur++;}int Find_hash(int key){    int temp=(key>0)?key:-key;    int cor_num=(temp)%MOD;    int u=hash_head[cor_num];    int coun=0;    while(u!=-1){        if(hash_[u].key==key){            coun++;        }        u=hash_[u].next;    }    return coun;}int main(){    while(~scanf("%d",&a[0])){        ///INI        cur=0;        for(int i=1;i<5;i++)            scanf("%d",&a[i]);        for(int i=0;i<MOD;i++){            //hash_[i].next=0;            //hash_[i].key=0;            hash_head[i]=-1;        }        ///SCANF        int key=0;        for(int i=-50;i<=50;i++){            if(i==0)    continue;            for(int j=-50;j<=50;j++){                if(j==0)    continue;                for(int k=-50;k<=50;k++){                    if(k==0)    continue;                    key=(a[0]*LF(i)+a[1]*LF(j)+a[2]*LF(k))*(-1);                    Insert_hash(key);                }            }        }        ///FIND        int res=0;        for(int i=-50;i<=50;i++){            if(i==0)    continue;            for(int j=-50;j<=50;j++){                if(j==0)    continue;                key=(a[3]*LF(i)+a[4]*LF(j));                res+=Find_hash(key);            }        }        cout<<res<<endl;    }}