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
", s);
    }
    return 0;
}