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
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
#include <bits/stdc++.h>


using namespace std;

const int MOD = 998'244'353;
const int ALPH = 7;


typedef array <int,ALPH> Vec;
typedef array <Vec,ALPH> Mat;

const Vec ZERO_VEC = {0, 0, 0, 0, 0, 0, 0};
const Mat ZERO_MAT = {ZERO_VEC, ZERO_VEC, ZERO_VEC, ZERO_VEC, ZERO_VEC, ZERO_VEC, ZERO_VEC};

Mat getIdenMat() {
    Mat iden = ZERO_MAT;
    for (int i = 0; i < ALPH; i++) {
        iden[i][i] = 1;
    }

    return iden;
}

const Mat IDEN_MAT = getIdenMat();

Mat operator*(const Mat &a, const Mat &b) {
    vector <vector<long long>> res(ALPH, vector <long long> (ALPH, 0));
    for (int i = 0; i < ALPH; i++) for (int j = 0; j < ALPH; j++) {
        res[i][0] += (long long) a[i][j] * b[j][0];
        res[i][1] += (long long) a[i][j] * b[j][1];
        res[i][2] += (long long) a[i][j] * b[j][2];
        res[i][3] += (long long) a[i][j] * b[j][3];
        res[i][4] += (long long) a[i][j] * b[j][4];
        res[i][5] += (long long) a[i][j] * b[j][5];
        res[i][6] += (long long) a[i][j] * b[j][6];
    }

    Mat resMod;
    for (int i = 0; i < ALPH; i++) for (int j = 0; j < ALPH; j++) {
        resMod[i][j] = res[i][j] % MOD;
    }

    return resMod;
}

Mat getTransForTotal(int c) {
    Mat trans = IDEN_MAT;
    for (int j = 0; j < ALPH; j++) {
        trans[j][c] = 1;
    }

    return trans;
}

Mat getTransForUnique(int i, int c, vector <set<int>> &pos) {
    auto it = pos[c].lower_bound(i);
    int iPrev = it == pos[c].begin() ? -1 : *prev(it);

    set <int> seen;
    for (int j = 0; j < ALPH - 1; j++) {
        auto it = pos[j].upper_bound(iPrev);
        if (it != pos[j].end() && *it <= i) {
            seen.insert(j);
        }
    }

    Mat trans = IDEN_MAT;

    trans[ALPH - 1][c] = iPrev == -1;
    for (int j : seen) {
        trans[j][c] = 1;
    }

    return trans;
}

int extractAnswerFromTrans(const Mat &transTotal, const Mat &transUnique) {
    int ans = 0;
    for (int j = 0; j < ALPH - 1; j++) {
        ans += transTotal[ALPH - 1][j];
        if (ans >= MOD) {
            ans -= MOD;
        }

        ans -= transUnique[ALPH - 1][j];
        if (ans < 0) {
            ans += MOD;
        }
    }

    return ans;
}

struct MatMulTree {
    Mat combine(const Mat &a, const Mat &b) { return a * b; }

    int p = 1;
    vector <Mat> tree;

    MatMulTree(int n, vector <Mat> &vec) {
        while (p < n) p *= 2;

        int m = 2 * p - 1;
        tree.resize(m);

        for (int i = p - 1; i < m; i++) {
            int iOrig = i - p + 1;
            if (iOrig < n) {
                tree[i] = vec[iOrig];
            } else {
                tree[i] = IDEN_MAT;
            }
        }

        for (int i = p - 2; i >= 0; i--) {
            tree[i] = combine(tree[2 * i + 1], tree[2 * i + 2]);
        }
    }

    int par(int x) { return (x - 1) / 2; }

    void update(int l, const Mat &v) {
        l += p - 1;
        tree[l] = v;

        while (l > 0) {
            l = par(l);
            tree[l] = combine(tree[2 * l + 1], tree[2 * l + 2]);
        }
    }
};

vector <int> solve(int n, int q, vector <int> &seq, vector <pair<int, int>> &ops) {
    vector <set<int>> pos(ALPH - 1);
    for (int i = 0; i < n; i++) {
        pos[seq[i]].insert(i);
    }

    vector <Mat> matsForTotal(n), matsForUnique(n);
    for (int i = 0; i < n; i++) {
        matsForTotal[i] = getTransForTotal(seq[i]);
        matsForUnique[i] = getTransForUnique(i, seq[i], pos);
    }

    MatMulTree treeForTotal(n, matsForTotal), treeForUnique(n, matsForUnique);

    auto getAnswer = [&]() {
        return extractAnswerFromTrans(treeForTotal.tree[0], treeForUnique.tree[0]);
    };

    vector <int> ans = {getAnswer()};

    for (auto &[i, x] : ops) {
        if (x != seq[i]) {
            pos[seq[i]].erase(i);
            pos[x].insert(i);
            seq[i] = x;

            treeForTotal.update(i, getTransForTotal(seq[i]));

            for (int j = 0; j < ALPH - 1; j++) {
                auto it = pos[j].lower_bound(i);
                if (it != pos[j].end()) {
                    int iUpdate = *it;
                    treeForUnique.update(iUpdate, getTransForUnique(iUpdate, seq[iUpdate], pos));

                    if (j == x) {
                        it++;
                        if (it != pos[j].end()) {
                            int iUpdate = *it;
                            treeForUnique.update(iUpdate, getTransForUnique(iUpdate, seq[iUpdate], pos));
                        }
                    }
                }
            }
        }

        ans.push_back(getAnswer());
    }

    return ans;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int n, q;
    cin >> n >> q;

    string s;
    cin >> s;

    vector <int> seq(n);
    for (int i = 0; i < n; i++) {
        seq[i] = s[i] - 'a';
    }

    vector <pair<int, int>> ops(q);
    for (auto &[i, x] : ops) {
        char c;
        cin >> i >> c; i--; x = c - 'a';
    }

    auto ans = solve(n, q, seq, ops);
    for (int x : ans) {
        cout << x << '\n';
    }

    return 0;
}