Problem p02763 (C++)

Buggy submission (status: Time Limit Exceeded):
#define _CRT_SECURE_NO_WARNINGS
#include <algorithm>
#include <functional>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <vector>
#define _USE_MATH_DEFINES
#include <bitset>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
using namespace std;
typedef long long ll;
#define rep(i, a, b) for (auto i = a; i < b; i++)
#define all(_x) _x.begin(), _x.end()
#define r_sort(_x) sort(_x.begin(), _x.end(), std::greater<int>())
#define vec_cnt(_a, _n) (upper_bound(all(_a), _n) - lower_bound(all(_a), _n))
#define vec_unique(_a) _a.erase(std::unique(all(_a)), _a.end());
#define vvec vector<vector<ll>>
ll gcd(ll a, ll b) { return a % b == 0 ? b : gcd(b, a % b); }
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }
#define INF 1 << 30
const int mod = 1000000007;
ll power(ll x, ll p) {
  ll a = 1;
  while (p > 0) {
    if (p % 2 == 0) {
      x *= x;
      p /= 2;
    } else {
      a *= x;
      p--;
    }
  }
  return a;
}
ll mpower(ll x, ll p) {
  ll a = 1;
  while (p > 0) {
    if (p % 2 == 0) {
      x = x * x % mod;
      p /= 2;
    } else {
      a = a * x % mod;
      p--;
    }
  }
  return a;
}
ll c(ll n, ll k) {
  ll a = 1;
  rep(i, 1, k) {
    a *= n - i + 1;
    a /= i;
  }
  return a;
}
ll mc(ll n, ll m) {
  ll k = 1, l = 1;
  rep(i, n - m + 1, n + 1) k = k * i % mod;
  rep(i, 1, m + 1) l = l * i % mod;
  l = mpower(l, mod - 2);
  return k * l % mod;
}

int main(void) {
  int n, q, a, b, c;
  char ch;
  string s;
  cin >> n >> s >> q;
  vector<set<int>> pos(26);
  rep(i, 0, n) { pos[s[i] - 'a'].insert(i + 1); }
  rep(i, 0, q) {
    cin >> a >> b;
    if (a == 1) {
      cin >> ch;
      rep(i2, 0, 26) {
        if (pos[i2].find(b) != pos[i2].end()) {
          pos[i2].erase(b);
          pos[ch - 'a'].insert(b);
          break;
        }
      }
    } else {
      int cnt = 0;
      cin >> c;
      rep(i2, 0, 26) {
        auto it = lower_bound(all(pos[i2]), b);
        if (it != pos[i2].end() && *it <= c)
          cnt++;
      }
      printf("%d\n", cnt);
    }
  }
  return 0;
}


Fixed submission:
#define _CRT_SECURE_NO_WARNINGS
#include <algorithm>
#include <functional>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <vector>
#define _USE_MATH_DEFINES
#include <bitset>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
using namespace std;
typedef long long ll;
#define rep(i, a, b) for (auto i = a; i < b; i++)
#define all(_x) _x.begin(), _x.end()
#define r_sort(_x) sort(_x.begin(), _x.end(), std::greater<int>())
#define vec_cnt(_a, _n) (upper_bound(all(_a), _n) - lower_bound(all(_a), _n))
#define vec_unique(_a) _a.erase(std::unique(all(_a)), _a.end());
#define vvec vector<vector<ll>>
ll gcd(ll a, ll b) { return a % b == 0 ? b : gcd(b, a % b); }
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }
#define INF 1 << 30
const int mod = 1000000007;
ll power(ll x, ll p) {
  ll a = 1;
  while (p > 0) {
    if (p % 2 == 0) {
      x *= x;
      p /= 2;
    } else {
      a *= x;
      p--;
    }
  }
  return a;
}
ll mpower(ll x, ll p) {
  ll a = 1;
  while (p > 0) {
    if (p % 2 == 0) {
      x = x * x % mod;
      p /= 2;
    } else {
      a = a * x % mod;
      p--;
    }
  }
  return a;
}
ll c(ll n, ll k) {
  ll a = 1;
  rep(i, 1, k) {
    a *= n - i + 1;
    a /= i;
  }
  return a;
}
ll mc(ll n, ll m) {
  ll k = 1, l = 1;
  rep(i, n - m + 1, n + 1) k = k * i % mod;
  rep(i, 1, m + 1) l = l * i % mod;
  l = mpower(l, mod - 2);
  return k * l % mod;
}

int main(void) {
  int n, q, a, b, c;
  char ch;
  string s;
  cin >> n >> s >> q;
  vector<set<int>> pos(26);
  rep(i, 0, n) { pos[s[i] - 'a'].insert(i + 1); }
  rep(i, 0, q) {
    cin >> a >> b;
    if (a == 1) {
      cin >> ch;
      rep(i2, 0, 26) {
        if (pos[i2].find(b) != pos[i2].end()) {
          pos[i2].erase(b);
          pos[ch - 'a'].insert(b);
          break;
        }
      }
    } else {
      int cnt = 0;
      cin >> c;
      rep(i2, 0, 26) {
        auto it = pos[i2].lower_bound(b);
        if (it != pos[i2].end() && *it <= c)
          cnt++;
      }
      printf("%d\n", cnt);
    }
  }
  return 0;
}