题目：Two Rabbits HDU - 4745
分析：
1、先把它当成直线而不是圆，求出各个区间的最长回文子序列 2、考虑圆环：将其分成两段 如图：
代码：
#include <bits/stdc++.h>
using namespace std
;
typedef long long ll
;
#define print(i) cout << "debug: " << i << endl
#define close() ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define mem(a, b) memset(a, b, sizeof(a))
const ll mod
= 1e9 + 7;
const int maxn
= 2e6;
const int inf
= 0x3f3f3f3f;
int n
;
int a
[2010];
int dp
[2010][2010];
int main()
{
while(cin
>> n
&& n
)
{
for(int i
= 1; i
<= n
; i
++) cin
>> a
[i
];
for(int i
= 1; i
<= n
; i
++) dp
[i
][i
] = 1;
for(int len
= 2; len
<= n
; len
++)
{
for(int i
= 1, j
= i
+ len
- 1; j
<= n
; i
++, j
++)
{
dp
[i
][j
] = max(dp
[i
+ 1][j
], dp
[i
][j
- 1]);
if(a
[i
] == a
[j
]) dp
[i
][j
] = max(dp
[i
][j
], 2 + dp
[i
+ 1][j
- 1]);
}
}
int res
= 0;
for(int i
= 1; i
<= n
; i
++) res
= max(res
, dp
[1][i
] + dp
[i
+ 1][n
]);
cout
<< res
<< endl
;
}
}
