1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define deb(...)
#define DBP(...)
using namespace std;
using   ll         = long long;
using   vi         = vector<int>;
using   pii        = pair<int, int>;
#define pb           push_back
#define mp           make_pair
#define x            first
#define y            second
#define rep(i, b, e) for (int i = (b); i < (e); i++)
#define each(a, x)   for (auto& a : (x))
#define all(x)       (x).begin(), (x).end()
#define sz(x)        int((x).size())

void run();

int main() {
	cin.sync_with_stdio(0); cin.tie(0);
	cout << fixed << setprecision(12);
	run();
	cout << flush; _Exit(0);
}

constexpr int MOD = 998'244'353;
constexpr int ALPHA = 7; // end marker + a,b,c,d,e,f

struct State {
	ll any[ALPHA] = {}; // c -> #{paths ending with c}
	ll tight[ALPHA][ALPHA] = {}; // (d,c) -> #{tight paths ending with c, which don't pass over {c,d} when extended}

	State() {
		any[0] = 1;
		rep(i, 0, ALPHA) tight[i][0] = 1;
	}

	ll get() const {
		ll ans = 0;
		rep(i, 0, ALPHA) ans += any[i] - tight[0][i];
		ans %= MOD;
		if (ans < 0) ans += MOD;
		return ans;
	}
};

enum CellType : uint8_t {
	KEEP = 0, ZERO = 1, SET = 2
};

struct Transition {
	ll any[ALPHA][ALPHA] = {}; // c -> direct coefficients
	CellType cells[ALPHA][ALPHA] = {}; // (d,c) -> tight cell state
	ll tight[ALPHA][ALPHA] = {}; // c -> coefficients for refs
	bool refMasks[ALPHA][ALPHA] = {}; // index -> column mask in state
	int refRows[ALPHA] = {}; // index -> row in state
	int numRefs = 0;

	Transition() {
		rep(i, 0, ALPHA) any[i][i] = 1;
	}

	void push(int c) {
		rep(i, 0, ALPHA) if (i != c) rep(j, 0, ALPHA) any[c][j] += any[i][j];
		rep(j, 0, ALPHA) any[c][j] %= MOD;

		ll vec[ALPHA] = {};
		bool newRef = 0;

		rep(i, 0, ALPHA) {
			if (cells[c][i] == KEEP) {
				refMasks[numRefs][i] = 1;
				newRef = 1;
			} else if (cells[c][i] == SET) {
				rep(j, 0, numRefs) vec[j] += tight[i][j];
			}
		}

		rep(i, 0, numRefs) {
			vec[i] %= MOD;
		}

		if (newRef) {
			vec[numRefs] = 1;
			refRows[numRefs++] = c;
		}

		memcpy(tight[c], vec, sizeof(vec));
		rep(i, 0, ALPHA) cells[c][i] = ZERO;
		rep(i, 0, ALPHA) cells[i][c] = SET;
	}

	void apply(State& s) const {
		ll newAny[ALPHA];
		rep(i, 0, ALPHA) {
			newAny[i] = 0;
			rep(j, 0, ALPHA) newAny[i] += any[i][j] * s.any[j];
			newAny[i] %= MOD;
		}
		memcpy(s.any, newAny, sizeof(newAny));

		ll refVals[ALPHA];
		rep(i, 0, numRefs) {
			refVals[i] = 0;
			rep(j, 0, ALPHA) if (refMasks[i][j]) refVals[i] += s.tight[refRows[i]][j];
			refVals[i] %= MOD;
		}

		rep(i, 0, ALPHA) {
			ll val = 0;
			rep(j, 0, numRefs) val += tight[i][j] * refVals[j];
			val %= MOD;
			rep(j, 0, ALPHA) {
				if (cells[j][i] == SET) {
					s.tight[j][i] = val;
				} else if (cells[j][i] == ZERO) {
					s.tight[j][i] = 0;
				}
			}
		}
	}
};

constexpr int BLOCK = 300;

string txt;
vector<State> states;
vector<Transition> transitions;

void rebuild(int i) {
	int begin = i*BLOCK, end = min(begin+BLOCK, sz(txt));
	transitions[i] = {};
	rep(j, begin, end) transitions[i].push(txt[j]-'a'+1);
}

ll updateFrom(int i) {
	State state = states[i];
	transitions[i].apply(state);
	while (++i < sz(states)) {
		states[i] = state;
		transitions[i].apply(state);
	}
	return state.get();
}

void run() {
	int n, q, p;
	cin >> n >> q >> txt;

	int numBlocks = (n+BLOCK-1) / BLOCK;
	states.resize(numBlocks);
	transitions.resize(numBlocks);

	rep(i, 0, sz(states)) rebuild(i);
	cout << updateFrom(0) << '\n';

	while (q--) {
		cin >> p;
		p--;
		cin >> txt[p];
		rebuild(p/BLOCK);
		cout << updateFrom(p/BLOCK) << '\n';
	}
}