51Nod 1202 子序列个数 ( dp

序列有关的dp 状态$dp[i]$定义为第$i$个数字时的 子序列状态有多少个

$dp[0] = 1$ 考虑状态转移方程: $dp[i] = dp[i-1]*2​$ $dp[i] = dp[i-1]*2-dp[j-1]​$

感觉这种题都很有模板的感觉TAT

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const int MAXN = (int)2e5+10;
const int mod = (int)1e9+7;

int arr[MAXN];
ll dp[MAXN];

int main(){
    int n;
    cin >> n;
    for(int i = 1; i <= n; ++i)
        cin >> arr[i];
    dp[0] = 1;
    map<int, int> mp;
    for(int i = 1; i <= n; ++i) {
        if(mp[arr[i]]) {
            int j = mp[arr[i]];
            dp[i] = (dp[i - 1] * 2 - dp[j - 1] + mod) % mod;
        }
        else {
            dp[i] = (dp[i-1]*2) % mod;
        }
        mp[arr[i]] = i;
    }
    cout << dp[n]-1 << endl;
    return 0;
}