Problem p02599 (C++)

Buggy submission (status: Time Limit Exceeded):
// Program to compute no. of different elements
// of ranges for different range queries
#include <bits/stdc++.h>
using namespace std;

// Used in frequency array (maximum value of an
// array element).
const int MAX = 1000000;

// Variable to represent block size. This is made
// global so compare() of sort can use it.
int block;

// Structure to represent a query range and to store
// index and result of a particular query range
struct Query {
  int L, R, index, result;
};

// Function used to sort all queries so that all queries
// of same block are arranged together and within a block,
// queries are sorted in increasing order of R values.
bool compare(Query x, Query y) {
  // Different blocks, sort by block.
  if (x.L / block != y.L / block)
    return x.L / block < y.L / block;

  // Same block, sort by R value
  return x.R < y.R;
}

// Function used to sort all queries in order of their
// index value so that results of queries can be printed
// in same order as of input
bool compare1(Query x, Query y) { return x.index < y.index; }

// calculate distinct elements of all query ranges.
// m is number of queries n is size of array a[].
void queryResults(int a[], int n, Query q[], int m) {
  // Find block size
  block = (int)sqrt(n);

  // Sort all queries so that queries of same
  // blocks are arranged together.
  sort(q, q + m, compare);

  // Initialize current L, current R and current
  // different elements
  int currL = 0, currR = 0;
  int curr_Diff_elements = 0;

  // Initialize frequency array with 0
  int freq[MAX] = {0};

  // Traverse through all queries
  for (int i = 0; i < m; i++) {

    // L and R values of current range
    int L = q[i].L, R = q[i].R;

    // Remove extra elements of previous range.
    // For example if previous range is [0, 3]
    // and current range is [2, 5], then a[0]
    // and a[1] are subtracted
    while (currL < L) {

      // element a[currL] is removed
      freq[a[currL]]--;
      if (freq[a[currL]] == 0)
        curr_Diff_elements--;

      currL++;
    }

    // Add Elements of current Range
    // Note:- during addition of the left
    // side elements we have to add currL-1
    // because currL is already in range
    while (currL > L) {
      freq[a[currL - 1]]++;

      // include a element if it occurs first time
      if (freq[a[currL - 1]] == 1)
        curr_Diff_elements++;

      currL--;
    }
    while (currR <= R) {
      freq[a[currR]]++;

      // include a element if it occurs first time
      if (freq[a[currR]] == 1)
        curr_Diff_elements++;

      currR++;
    }

    // Remove elements of previous range. For example
    // when previous range is [0, 10] and current range
    // is [3, 8], then a[9] and a[10] are subtracted
    // Note:- Basically for a previous query L to R
    // currL is L and currR is R+1. So during removal
    // of currR remove currR-1 because currR was
    // never included
    while (currR > R + 1) {

      // element a[currL] is removed
      freq[a[currR - 1]]--;

      // if occurrence of a number is reduced
      // to zero remove it from list of
      // different elements
      if (freq[a[currR - 1]] == 0)
        curr_Diff_elements--;

      currR--;
    }
    q[i].result = curr_Diff_elements;
  }
}

// print the result of all range queries in
// initial order of queries
void printResults(Query q[], int m) {
  sort(q, q + m, compare1);
  for (int i = 0; i < m; i++) {
    cout << q[i].result << endl;
  }
}

// Driver program
int main() {
  int n, m;
  cin >> n >> m;
  int a[n];
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  Query q[m];
  for (int i = 0; i < m; ++i) {
    cin >> q[i].L >> q[i].R;
    q[i].L--;
    q[i].R--;
    q[i].index = i;
    q[i].result = 0;
  }
  queryResults(a, n, q, m);
  printResults(q, m);
  return 0;
}


Fixed submission:
// Program to compute no. of different elements
// of ranges for different range queries
#include <bits/stdc++.h>
using namespace std;

// Used in frequency array (maximum value of an
// array element).
const int MAX = 1000000;

// Variable to represent block size. This is made
// global so compare() of sort can use it.
int block;

// Structure to represent a query range and to store
// index and result of a particular query range
struct Query {
  int L, R, index, result;
};

// Function used to sort all queries so that all queries
// of same block are arranged together and within a block,
// queries are sorted in increasing order of R values.
bool compare(Query x, Query y) {
  // Different blocks, sort by block.
  if (x.L / block != y.L / block)
    return x.L / block < y.L / block;

  // Same block, sort by R value
  return x.R < y.R;
}

// Function used to sort all queries in order of their
// index value so that results of queries can be printed
// in same order as of input
bool compare1(Query x, Query y) { return x.index < y.index; }

// calculate distinct elements of all query ranges.
// m is number of queries n is size of array a[].
void queryResults(int a[], int n, Query q[], int m) {
  // Find block size
  block = 800;

  // Sort all queries so that queries of same
  // blocks are arranged together.
  sort(q, q + m, compare);

  // Initialize current L, current R and current
  // different elements
  int currL = 0, currR = 0;
  int curr_Diff_elements = 0;

  // Initialize frequency array with 0
  int freq[MAX] = {0};

  // Traverse through all queries
  for (int i = 0; i < m; i++) {

    // L and R values of current range
    int L = q[i].L, R = q[i].R;

    // Remove extra elements of previous range.
    // For example if previous range is [0, 3]
    // and current range is [2, 5], then a[0]
    // and a[1] are subtracted
    while (currL < L) {

      // element a[currL] is removed
      freq[a[currL]]--;
      if (freq[a[currL]] == 0)
        curr_Diff_elements--;

      currL++;
    }

    // Add Elements of current Range
    // Note:- during addition of the left
    // side elements we have to add currL-1
    // because currL is already in range
    while (currL > L) {
      freq[a[currL - 1]]++;

      // include a element if it occurs first time
      if (freq[a[currL - 1]] == 1)
        curr_Diff_elements++;

      currL--;
    }
    while (currR <= R) {
      freq[a[currR]]++;

      // include a element if it occurs first time
      if (freq[a[currR]] == 1)
        curr_Diff_elements++;

      currR++;
    }

    // Remove elements of previous range. For example
    // when previous range is [0, 10] and current range
    // is [3, 8], then a[9] and a[10] are subtracted
    // Note:- Basically for a previous query L to R
    // currL is L and currR is R+1. So during removal
    // of currR remove currR-1 because currR was
    // never included
    while (currR > R + 1) {

      // element a[currL] is removed
      freq[a[currR - 1]]--;

      // if occurrence of a number is reduced
      // to zero remove it from list of
      // different elements
      if (freq[a[currR - 1]] == 0)
        curr_Diff_elements--;

      currR--;
    }
    q[i].result = curr_Diff_elements;
  }
}

// print the result of all range queries in
// initial order of queries
void printResults(Query q[], int m) {
  sort(q, q + m, compare1);
  for (int i = 0; i < m; i++) {
    cout << q[i].result << endl;
  }
}

// Driver program
int main() {
  int n, m;
  cin >> n >> m;
  int a[n];
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  Query q[m];
  for (int i = 0; i < m; ++i) {
    cin >> q[i].L >> q[i].R;
    q[i].L--;
    q[i].R--;
    q[i].index = i;
    q[i].result = 0;
  }
  queryResults(a, n, q, m);
  printResults(q, m);
  return 0;
}