递推DP HDOJ 5328 Problem Killer

题目传送门

 /*
     递推DP: 如果a, b, c是等差数列，且b, c, d是等差数列，那么a, b, c, d是等差数列，等比数列同理
             判断ai-2, ai-1, ai是否是等差(比)数列，能在O(n)时间求出最长的长度
 */
 #include <cstdio>
 #include <algorithm>
 #include <cstring>
 #include <cmath>
 using namespace std;

 typedef long long ll;
 const int MAXN = 1e6 + ;
 const int INF = 0x3f3f3f3f;
 ll a[MAXN];
 int dp1[MAXN], dp2[MAXN];

 int main(void)  {       //HDOJ 5328 Problem Killer
     int T;  scanf ("%d", &T);
     while (T--) {
         int n;  scanf ("%d", &n);
         for (int i=; i<=n; ++i)    scanf ("%I64d", &a[i]);
         if (n <= ) printf ("%d\n", n);
         else    {
             for (int i=; i<=n; ++i)    dp1[i] = dp2[i] = ;
             int ans = ;
             for (int i=; i<=n; ++i)    {
                 if (a[i-] + a[i] == a[i-] * )    dp1[i] = dp1[i-] + ;
                 if (a[i-] * a[i] == a[i-] * a[i-])   dp2[i] = dp2[i-] + ;
                 ans = max (ans, max (dp1[i], dp2[i]));
             }
             printf ("%d\n", ans);
         }
     }

     return ;
 }