Palindrome Bo (预处理 + 区间DP)

先进行离散化，然后再预处理出所有位置的下一个元素，做好这一步对时间的优化非常重要。

剩下的就是一般的DP了。区间DP

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

const int maxn = ;
const ll mod = 1e9 + ;

ll dp[maxn][maxn], f[maxn][maxn], a[maxn];
int nex[maxn][maxn], pre[maxn][maxn];
pair<ll, int>discre[maxn];

int main(){
    int n;while(~scanf("%d", &n)){
        for(int i = ; i <= n; i ++){
            scanf("%lld", &a[i]);
            discre[i].first = a[i];
            discre[i].second = i;
        } sort(discre + , discre +  + n);
        discre[].first = -;
        int cnt = ;
        for(int i = ; i <= n; i ++){
            if(discre[i].first != discre[i - ].first) cnt ++;
            a[discre[i].second] = cnt;
        }
        a[] = a[n + ] = cnt + ;
        memset(dp, -, sizeof(dp));
        memset(nex, , sizeof(nex));
        memset(pre, , sizeof(pre));
        for(int i = ; i <= n + ; i ++){
            for(int j = i + ; j <= n + ; j ++)
                if(!nex[i][a[j]]) nex[i][a[j]] = j;
            for(int j = i - ; j >= ; j --)
                if(!pre[i][a[j]]) pre[i][a[j]] = j;
        }
        for(int l = n + ; l >= ; l -- ){
            ll ans1 = , ans2 = ;
            for(int r = l; r <= n + ; r ++){
                ll tmp1, tmp2;
                if(l == r){
                    dp[l][r] = f[l][r] = ;
                    continue;
                }

                if(a[r - ] <= a[l]){
                    tmp1 = dp[nex[l][a[r-]]][pre[r-][a[r-]]];
                    tmp2 =  f[nex[l][a[r-]]][pre[r-][a[r-]]];
                    if(ans1 == tmp1) ans2 = (ans2 - tmp2 + mod) % mod;
                    if(ans1 < tmp1) ans1 = tmp1, ans2 = tmp2;
                    tmp1 = dp[nex[l][a[r-]]][r - ];
                    tmp2 =  f[nex[l][a[r-]]][r - ];
                    if(ans1 == tmp1) ans2 = (ans2 + tmp2) % mod;
                    if(ans1 < tmp1) ans1 = tmp1, ans2 = tmp2;
                }

                if(a[l] == a[r]){
                    dp[l][r] = ans1 + ;
                    f[l][r] = ans2;
                }else dp[l][r] = f[l][r] = ;
            }
        }
        ll ans1 = , ans2 = ;
        for(int i = ; i <= cnt; i ++){
            if(dp[nex[][i]][pre[n+][i]] > ans1){
                ans1 = dp[nex[][i]][pre[n+][i]];
                ans2 =  f[nex[][i]][pre[n+][i]];
            }else if(ans1 == dp[nex[][i]][pre[n+][i]])
                ans2 = (ans2 + f[nex[][i]][pre[n+][i]]) % mod;
        }
        printf("%lld %lld\n",ans1, ans2);
    }
    return ;
}