题目传送门
1 /*
2 题意：一只兔子顺时针跳，另一只逆时针跳，跳石头权值相等而且不能越过起点
3 LPS：这道就是LPS的应用，把环倍增成链，套一下LPS，然而并不能理解dp[i][i+n-2] + 1，看别人的解题报告吧，以后来补(玩游戏)
4 详细解释
5 */
6 /************************************************
7 * Author :Running_Time
8 * Created Time :2015-8-8 16:57:23
9 * File Name :HDOJ_4747_LPS.cpp
10 ************************************************/
11
12 #include <cstdio>
13 #include <algorithm>
14 #include <iostream>
15 #include <sstream>
16 #include <cstring>
17 #include <cmath>
18 #include <string>
19 #include <vector>
20 #include <queue>
21 #include <deque>
22 #include <stack>
23 #include <list>
24 #include <map>
25 #include <set>
26 #include <bitset>
27 #include <cstdlib>
28 #include <ctime>
29 using namespace std;
30
31 #define lson l, mid, rt << 1
32 #define rson mid + 1, r, rt << 1 | 1
33 typedef long long ll;
34 const int MAXN = 2e3 + 10;
35 const int INF = 0x3f3f3f3f;
36 const int MOD = 1e9 + 7;
37 int a[MAXN], dp[MAXN][MAXN];
38
39
40 int main(void) { //HDOJ 4745 Two Rabbits
41 int n;
42 while (scanf ("%d", &n) == 1) {
43 if (!n) break;
44 for (int i=1; i<=n; ++i) {
45 scanf ("%d", &a[i]);
46 a[i+n] = a[i];
47 }
48 memset (dp, 0, sizeof (dp));
49 for (int i=1; i<=2*n; ++i) dp[i][i] = 1;
50 for (int i=2; i<2*n; ++i) {
51 for (int j=1; j+i-1<=2*n; ++j) {
52 if (a[j] == a[j+i-1]) dp[j][j+i-1] = dp[j+1][j+i-2] + 2;
53 else {
54 dp[j][j+i-1] = max (dp[j+1][j+i-1], dp[j][j+i-2]);
55 }
56 }
57 }
58 int ans = 0;
59 for (int i=1; i<=n; ++i) {
60 ans = max (ans, dp[i][i+n-1]);
61 ans = max (ans, dp[i][i+n-2] + 1);
62 }
63 printf ("%d
", ans);
64 }
65
66 return 0;
67 }