hdu 5400 Arithmetic Sequence（重构数组）

Arithmetic Sequence

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 65536/65536 K (Java/Others)
Total Submission(s): 1430    Accepted Submission(s): 625

Problem Description

A sequence b1,b2,⋯,bn are called (d1,d2)-arithmetic sequence if and only if there exist i(1≤i≤n) such that for every j(1≤j<i),bj+1=bj+d1 and for every j(i≤j<n),bj+1=bj+d2.

Teacher Mai has a sequence a1,a2,⋯,an. He wants to know how many intervals [l,r](1≤l≤r≤n) there are that al,al+1,⋯,ar are (d1,d2)-arithmetic sequence.

 

Input

There are multiple test cases.

For each test case, the first line contains three numbers n,d1,d2(1≤n≤105,|d1|,|d2|≤1000), the next line contains n integers a1,a2,⋯,an(|ai|≤109).

 

Output

For each test case, print the answer.

 

Sample Input

5 2 -20 2 0 -2 05 2 32 3 3 3 3

 

Sample Output

125

题意：求一个区间，使得区间内存在一点i，序列[l,i)是公差为d1的等差数列，序列[i,r]是公差为d2的等差数列，问你一共有几个区间满足

思路：枚举i点即可，但是直接枚举会有O（n²）的复杂度会炸掉，所以要用l和r数组分别记录当前点最大能延伸到前面和后面的距离，最后相乘即可，注意两点

1：当d1=d2的时候，区间会算重，此时只需要特判即可，值等于所有l或者所有r

2：l和r数组必须为long long 10^6*10^6会超出int型....因为这个WA了好多次。

代码：

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <cmath>using namespace std;#define N 100010#define LL long longint a[N];LL l[N],r[N];int main(){    int n,d1,d2;    while(~scanf("%d %d %d",&n,&d1,&d2))    {        long long ans=0;        for(int i=1; i<=n; i++)        {            scanf("%d",&a[i]);            l[i]=r[i]=1;        }        for(int i=2; i<=n; i++)            if(a[i]==a[i-1]+d1)                l[i]=l[i-1]+1;        for(int i=n-1; i>=1; i--)            if(a[i+1]==a[i]+d2)                r[i]=r[i+1]+1;        if(d1==d2)        {            for(int i=1; i<=n; i++)                ans+=l[i];        }        else        {            for(int i=1; i<=n; i++)                ans+=l[i]*r[i];        }        printf("%lld\n",ans);    }    return 0;}