求两个串的公共子串的个数，LCS变形

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long ll;
const int MAXN = 1010;
const int mod = 1000000007;
int N, M, a[MAXN], b[MAXN];
ll dp[MAXN][MAXN];
int main() {
    while (~scanf("%d %d", &N, &M)) {
        for (int i = 1; i <= N; i++) scanf("%d", &a[i]);
        for (int j = 1; j <= M; j++) scanf("%d", &b[j]);
        memset(dp, 0, sizeof(dp));
        for (int i = 1; i <= N; i++) {
            for (int j = 1; j <= M; j++) {
                if (a[i] == b[j]) dp[i][j] = dp[i - 1][j] + dp[i][j - 1] + 1;
                else dp[i][j] = dp[i - 1][j] + dp[i][j - 1] - dp[i - 1][j - 1] + mod;
                dp[i][j] %= mod;
            }
        }
        printf("%I64d\n", dp[N][M]);
    }
    return 0;
}