hdu 4745 Two Rabbits

构造回文串，求出[i, i + n]之间的最大回文串长度，最大回文串长度与前一字符串长度比较就是了

 

#pragma comment(linker,"/STACK:1024000000,1024000000")

#include <cstdio>



int n;

int num[2005];

int dp[2005][2005];



int max(int x, int y)

{

	return x > y ? x :y;

}



int dfs(int l, int r)

{

	if(dp[l][r]) return dp[l][r];

	if(l == r) return dp[l][r] = 1;

	if(l > r) return 0;

	dp[l][r] = max(dfs(l, r - 1), dfs(l + 1, r));

    int v = dfs(l + 1, r - 1);

	if(r - l == n) --v;

	if(num[l] == num[r]) v += 2;

    dp[l][r] = max(dp[l][r], v);

	return dp[l][r];

}



int main()

{

	//freopen("in.txt", "r", stdin);

    while(scanf("%d", &n) != EOF)

    {

        if(!n) break;

        for(int i = 2 * n; i >= 0; --i)

        	for(int j = 2 * n; j >= 0; --j)

        		dp[i][j] = 0;

        for(int i  = 0; i < n; ++i)

        {

        	scanf("%d", &num[i]);

        	num[n + i] = num[i];

        }

        int s = 0;

        for(int i = 0; i < n; ++i)

        	s = max(s, dfs(i, i + n));

        printf("%d\n", s);

    }

    return 0;

}