hdu4757 最长回文子序列（区间DP）

http://acm.hdu.edu.cn/showproblem.php?pid=4745

Problem Description

Long long ago, there lived two rabbits Tom and Jerry in the forest. On a sunny afternoon, they planned to play a game with some stones. There were n stones on the ground and they were arranged as a clockwise ring. That is to say, the first stone was adjacent
to the second stone and the n-th stone, and the second stone is adjacent to the first stone and the third stone, and so on. The weight of the i-th stone is ai.

The rabbits jumped from one stone to another. Tom always jumped clockwise, and Jerry always jumped anticlockwise.

At the beginning, the rabbits both choose a stone and stand on it. Then at each turn, Tom should choose a stone which have not been stepped by itself and then jumped to it, and Jerry should do the same thing as Tom, but the jumping direction is anti-clockwise.

For some unknown reason, at any time , the weight of the two stones on which the two rabbits stood should be equal. Besides, any rabbit couldn‘t jump over a stone which have been stepped by itself. In other words, if the Tom had stood on the second stone, it
cannot jump from the first stone to the third stone or from the n-the stone to the 4-th stone.

Please note that during the whole process, it was OK for the two rabbits to stand on a same stone at the same time.

Now they want to find out the maximum turns they can play if they follow the optimal strategy.

Input

The input contains at most 20 test cases.

For each test cases, the first line contains a integer n denoting the number of stones.

The next line contains n integers separated by space, and the i-th integer ai denotes the weight of the i-th stone.(1 <= n <= 1000, 1 <= ai <= 1000)

The input ends with n = 0.

Output

For each test case, print a integer denoting the maximum turns.

Sample Input

1
1
4
1 1 2 1
6
2 1 1 2 1 3
0

Sample Output

1
4
5

Hint

For the second case, the path of the Tom is 1, 2, 3, 4, and the path of Jerry is 1, 4, 3, 2.
For the third case, the path of Tom is 1,2,3,4,5 and the path of Jerry is 4,3,2,1,5.

/**
hdu 4745   最长回文子序列  （区间DP）
题目大意：在一个长度为n的环形序列上的任取两个点，一个向左走一个向右走，求他们走了多少步后二人可以再同一个点相遇（题目数据保证他们会在一个点相遇）
解题思路：看了题解我才明白，其实就是求这个环形序列的最长回文子序列，然后考虑两个起点的值相同时的情况就好了
          1、起点相同 maxx=max(maxx,dp[i][i+n-1]);
          2、地点不同  maxx=max(maxx,dp[i][i+n-2]+1);
*/
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <iostream>
using namespace std;
int dp[2005][2005],a[2005],n;
int main()
{
    while(~scanf("%d",&n))
    {
        if(n==0) break;
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i]);
            a[i+n]=a[i];
        }
        n*=2;
        memset(dp,0,sizeof(dp));
        for(int i=0;i<=n;i++)
            dp[i][i]=1;
        for(int i=n-1;i>0;i--)
        {
            for(int j=i+1;j<=n;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],dp[i+1][j-1]+2);
                }
            }
        }
        /**
        for(int i=1;i<=n;i++)
        {
            for(int j=1;j<=n;j++)
            {
                printf("%d ",dp[i][j]);
            }
            printf("\n");
        }*/
        int maxx=-1;
        n/=2;
        for(int i=1;i<=n;i++)
        {
            maxx=max(maxx,dp[i][i+n-1]);
        }
        ///如果二者各自起始点石头重量相同
        for(int i=1;i<=n;i++)
        {
            maxx=max(maxx,dp[i][i+n-2]+1);
        }
        printf("%d\n",maxx);
    }
    return 0;
}