Language: C++

#include <iostream>
#include <string>
#include <cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#define _USE_MATH_DEFINES
#include <math.h>
#include <functional>
#include<complex>
#include<cassert>
using namespace std;
#include<atcoder/all>
using namespace atcoder;


#define rep(i,x) for(ll i=0;i<x;i++)
#define repn(i,x) for(ll i=1;i<=x;i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
	if (a > b) { return a; }
	return b;
}

ll min(ll a, ll b) {
	if (a > b) { return b; }
	return a;
}

ll gcd(ll a, ll b) {
	if (b == 0) { return a; }
	if (a < b) { return gcd(b, a); }
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

struct edge {
	ll ind;
	ll fr;
	ll to;
	ll d;
};

class mint {
	long long x;
public:
	mint(long long x = 0) : x((x% MOD + MOD) % MOD) {}
	mint operator-() const {
		return mint(-x);
	}
	mint& operator+=(const mint& a) {
		if ((x += a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator-=(const mint& a) {
		if ((x += MOD - a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator*=(const  mint& a) {
		(x *= a.x) %= MOD;
		return *this;
	}
	mint operator+(const mint& a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint& a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint& a) const {
		mint res(*this);
		return res *= a;
	}

	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime MOD
	mint inv() const {
		return pow(MOD - 2);
	}
	mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint& a) const {
		mint res(*this);
		return res /= a;
	}

	friend ostream& operator<<(ostream& os, const mint& m) {
		os << m.x;
		return os;
	}
};

mint pw(mint a, ll b) {
	if (b == 0) { return 1; }
	mint ret = pw(a, b >> 1);
	ret *= ret;
	if (b & 1) { ret *= a; }
	return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

/////////////////////////////////////


int main() {
	string S;
	cin >> S;

	
	ll N = S.size();
	
	vll a(N + 1);
	repn(i, N) {
		if (S[i - 1] == 'a') { a[i] = 1; }
		else { a[i] = 2; }
	}

	vll sum(N + 1, 0);
	vvll lis(3);

	repn(i, N) {
		sum[i] = (sum[i - 1] + a[i]) % 3;
		lis[sum[i]].push_back(i);
	}

	vvll ad(N + 1);
	ll ren = N + 1;
	for (ll i = N-1; i >= 0; i--) {
		if (i <= N - 2 && a[i + 1] == a[i + 2]) { ren = i + 2; }

		ad[i + 1].push_back(i);

		ll x = (sum[i] + (3-a[i+1])) % 3;
		auto itr = lower_bound(lis[x].begin(), lis[x].end(), ren);
		
		if (itr != lis[x].end()) { ad[*itr].push_back(i); }
	}

	if (ren == N + 1) { cout << 1 << endl; return 0; }

	vmint dp(N + 1, 0);
	dp[0] = 1;


	repn(i, N) {
		//cout << i << endl;
		for (ll j : ad[i]) {
			dp[i] += dp[j];
			//cout << j << " ";
		}
		//cout << endl;
	}

	mint ans = 0;
	repn(i, N) {
		if (sum[i] == sum[N]) { ans += dp[i]; }
	}

	cout << ans << endl;
	
}


Compiler output:
a.cc:16:9: fatal error: atcoder/all: No such file or directory
   16 | #include<atcoder/all>
      |         ^~~~~~~~~~~~~
compilation terminated.