Problem:
Score : 500 points

Problem Statement

You are given two integer sequences S and T of length N and M, respectively, both consisting of integers between 1 and 10^5 (inclusive).

In how many pairs of a subsequence of S and a subsequence of T do the two subsequences are the same in content?

Here the subsequence of A is a sequence obtained by removing zero or more elements from A and concatenating the remaining elements without changing the order.

For both S and T, we distinguish two subsequences if the sets of the indices of the removed elements are different, even if the subsequences are the same in content.

Since the answer can be tremendous, print the number modulo 10^9+7.

Constraints

1 \leq N, M \leq 2 \times 10^3

The length of S is N.

The length of T is M.

1 \leq S_i, T_i \leq 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
S_1 S_2 ... S_{N-1} S_{N}
T_1 T_2 ... T_{M-1} T_{M}

Output

Print the number of pairs of a subsequence of S and a subsequence of T such that the subsequences are the same in content, modulo 10^9+7.

Sample Input 1

2 2
1 3
3 1

Sample Output 1

3

S has four subsequences: (), (1), (3), (1, 3).

T has four subsequences: (), (3), (1), (3, 1).

There are 1 \times 1 pair of subsequences in which the subsequences are both (), 1 \times 1 pair of subsequences in which the subsequences are both (1), and 1 \times 1 pair of subsequences in which the subsequences are both (3), for a total of three pairs.

Sample Input 2

2 2
1 1
1 1

Sample Output 2

6

S has four subsequences: (), (1), (1), (1, 1).

T has four subsequences: (), (1), (1), (1, 1).

There are 1 \times 1 pair of subsequences in which the subsequences are both (), 2 \times 2 pairs of subsequences in which the subsequences are both (1), and 1 \times 1 pair of subsequences in which the subsequences are both (1,1), for a total of six pairs.
Note again that we distinguish two subsequences if the sets of the indices of the removed elements are different, even if the subsequences are the same in content.

Sample Input 3

4 4
3 4 5 6
3 4 5 6

Sample Output 3

16

Sample Input 4

10 9
9 6 5 7 5 9 8 5 6 7
8 6 8 5 5 7 9 9 7

Sample Output 4

191

Sample Input 5

20 20
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Sample Output 5

846527861

Be sure to print the number modulo 10^9+7.

Solution in Rust (1.15.1) (judge verdict: Accepted)

fn main() {
    let (n,m) = read_pair();
    let s: Vec<i32> = read_vec();
    let t: Vec<i32> = read_vec();

    let mut dp = vec![vec![F(0); m]; n];
    let mut ds = vec![vec![F(0); m+1]; n+1];

    for i in 0..n { for j in 0..m {
        if s[i] == t[j] {
            dp[i][j] = F(1);
            if i!=0 && j!=0 {
                dp[i][j] += ds[i][j];
            }
        }
        ds[i+1][j+1] = ds[i][j+1] + ds[i+1][j] - ds[i][j] + dp[i][j];
    }}

    println!("{}", dp.iter().fold(F(0), |s,v| s + v.iter().sum())+F(1));
}

fn get_line() -> String {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).ok();
    buf.trim().to_owned()
}
fn read_vec<T: std::str::FromStr>() -> Vec<T>{
    get_line().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}
fn read_pair<T: std::str::FromStr + Copy>() -> (T,T){
    let v:Vec<T> = read_vec(); (v[0], v[1])
}

const MOD: u64 = 1_000_000_007;

#[derive(Copy, Clone, PartialEq, Eq)]
struct F(u64);

impl F {
    pub fn new(n: i32) -> Self { F((MOD as i32 + n)as u64%MOD) }
    pub fn pow(self, mut exp: u32) -> Self {
        let mut base = self.0;
        let mut acc = 1;
        while exp > 1 {
            if (exp & 1) == 1 { acc *= base; acc %= MOD; }
            exp /= 2; base *= base; base %= MOD;
        }
        if exp == 1 { acc *= base;acc %= MOD; }
        F(acc)
    }
}
impl std::fmt::Display for F {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}
use std::ops::*;
impl Mul for F {
    type Output = Self;
    fn mul(self, rhs: Self) -> Self {F((self.0 * rhs.0)%MOD)}
}
impl MulAssign for F {
    fn mul_assign(&mut self, rhs: Self) {*self = F((self.0 * rhs.0)%MOD)}
}
impl Add for F {
    type Output = Self;
    fn add(self, rhs: Self) -> Self {F((self.0 + rhs.0)%MOD)}
}
impl AddAssign for F {
    fn add_assign(&mut self, rhs: Self) {*self = F((self.0 + rhs.0)%MOD)}
}
impl Neg for F {
    type Output = Self;
    fn neg(self) -> Self {if self.0==0 {F(0)} else {F(MOD - self.0)}}
}
impl Sub for F {
    type Output = Self;
    fn sub(self, rhs: Self) -> Self {F((MOD + self.0 - rhs.0)%MOD)}
}
impl SubAssign for F {
    fn sub_assign(&mut self, rhs: Self) {*self = F((MOD + self.0 - rhs.0)%MOD)}
}
impl Div for F {
    type Output = Self;
    fn div(self, rhs: Self) -> Self {F(self.0 * rhs.pow(MOD as u32-2).0 %MOD)}
}
impl DivAssign for F {
    fn div_assign(&mut self, rhs: Self) { *self = F(self.0 * rhs.pow(MOD as u32 - 2).0 % MOD) }
}
impl std::iter::Sum<F> for F {
    fn sum<I: Iterator<Item=F>>(iter: I) -> F { iter.fold(F(0), |a,b| a + b) }
}
impl<'a> std::iter::Sum<&'a F> for F {
    fn sum<I: Iterator<Item=&'a F>>(iter: I) -> F { iter.fold(F(0), |a, b| a + *b) }
}