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
#include <bits/stdc++.h>
using namespace std;

#define loop(i, a, b) for(int i = a; i <= b; i++)
#define loop_rev(i, a, b) for(int i = a; i >= b; i--)
#define all(x) x.begin(), x.end()
#define sz(x) int(x.size())
#define pb push_back

using ll = long long;

void solve();
vector<int> s;
int n, q;

vector<vector<vector<vector<int>>>> dp;
int maxi = 0;

signed main() {
  cin.tie(0)->sync_with_stdio(0);
  string str; cin >> n >> q >> str;
  dp.resize(n + 1, vector(n + 1, vector(6, vector(6, int(0)))));
  s.resize(n + 1);

  multiset<int> colors;

  loop(i, 1, n) {
    s[i] = (str[i - 1] - 'a');
    colors.insert(s[i]);
  }
  
  maxi = *prev(colors.end());
  solve();

  loop(qi, 1, q) {
    int i; char c; cin >> i >> c;
    colors.erase(colors.find(s[i]));
    s[i] = (c - 'a');
    colors.insert(s[i]);
    maxi = *prev(colors.end());
    solve();
  }

}

constexpr int MOD = 998'244'353;

ll add(ll a, ll b) { return (a + b >= MOD ? a + b - MOD : a + b); }
ll sub(ll a, ll b) { return (a - b < 0 ? a - b + MOD : a - b); }
ll mul(ll a, ll b) { return (a * b) % MOD; }

vector<vector<int>> nast, pop;

int rec(int a, int b, int c1, int c2) {
  int& res = dp[a][b][c1][c2];
  if(res) return (res - 1);
  if(a == b) {
    return (res = 2) - 1;
  }
  int lim1 = nast[c1][a];
  int lim2 = pop[c2][b];
  res = 1;
  if((!lim1 || lim1 >= b) && lim2 <= a) {
    ++res;
  }
  loop(nc1, 0, maxi) {
    int x = nast[nc1][a];
    if(!x || (lim1 && x > lim1)) continue;
    loop(nc2, 0, maxi) {
      int y = pop[nc2][b];
      if(x > y || (lim2 && y < lim2)) continue;
      int new_add = rec(x, y, nc1, nc2);
      res = add(res, new_add);
    }
  }
  return (res - 1);
}

void solve() {
  nast.resize(6, vector<int>(n + 2));
  pop.resize(6, vector<int>(n + 2));

  loop(i, 1, n) {
    loop(j, i, n) {
      loop(c1, 0, maxi) {
        loop(c2, 0, maxi) {
          dp[i][j][c1][c2] = 0;
        }
      }
    }
  }

  loop(i, 1, n) {
    loop(c, 0, maxi) {
      if(s[i] == c) {
        pop[c][i + 1] = i;
      }
      else {
        pop[c][i + 1] = pop[c][i];
      }
    }
  }

  loop_rev(i, n, 1) {
    loop(c, 0, maxi) {
      if(s[i] == c) {
        nast[c][i - 1] = i;
      }
      else {
        nast[c][i - 1] = nast[c][i];
      }
    }
  }

  int res = 0;

  loop(c1, 0, maxi) {
    int a = nast[c1][0];
    if(!a) continue;
    loop(c2, 0, maxi) {
      int b = pop[c2][n + 1];
      if(a > b) continue;
      int new_add = rec(a, b, c1, c2);
      res = add(res, new_add);
    }
  }

  vector<int> cnt_arr(6);
  int cnt = 1;

  loop(i, 1, n) {
    int old_cnt = cnt;
    cnt = sub(mul(cnt, 2), cnt_arr[s[i]]);
    cnt_arr[s[i]] = old_cnt;
  }

  cout << sub(cnt, add(res, 1)) << '\n';

}