Count subarrays for every array element in which they are the minimum

Last Updated : 21 Apr, 2021

Given an array arr[] consisting of N integers, the task is to create an array brr[] of size N where brr[i] represents the count of subarrays in which arr[i] is the smallest element.

Examples:

Input: arr[] = {3, 2, 4}
Output: {1, 3, 1}
Explanation:
For arr[0], there is only one subarray in which 3 is the smallest({3}).
For arr[1], there are three such subarrays where 2 is the smallest({2}, {3, 2}, {2, 4}).
For arr[2], there is only one subarray in which 4 is the smallest({4}).
Input: arr[] = {1, 2, 3, 4, 5}
Output: {5, 4, 3, 2, 1}

Naive Approach: The simplest approach is to generate all subarrays of the given array and for every array element arr[i], count the number of subarrays in which it is the smallest element.

Time Complexity: O(N³)
Auxiliary Space: O(N)

Efficient Approach: To optimize the above approach, the idea is to find the boundary index for every element, up to which it is the smallest element. For each element let L and R be the boundary indices on the left and right side respectively up to which arr[i] is the minimum. Therefore, the count of all subarrays can be calculated by:

(L + R + 1)*(R + 1)

Follow the steps below to solve the problem:

Store all the indices of array elements in a Map.
Sort the array in increasing order.
Initialize an array boundary[].
Iterate over the sorted array arr[] and simply insert the index of that element using Binary Search. Suppose it got inserted at index i, then its left boundary is boundary[i – 1] and its right boundary is boundary[i + 1].
Now, using the above formula, find the number of subarrays and keep track of that count in the resultant array.
After completing the above steps, print all the counts stored in the resultant array.

Below is the implementation of the above approach:

C++14

// C++14 program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the boundary of every
// element within which it is minimum
int binaryInsert(vector<int> &boundary, int i)
{
    int l = 0;
    int r = boundary.size() - 1;
 
    // Perform Binary Search
    while (l <= r)
    {
         
        // Find mid m
        int m = (l + r) / 2;
 
        // Update l
        if (boundary[m] < i)
            l = m + 1;
 
        // Update r
        else
            r = m - 1;
    }
 
    // Inserting the index
    boundary.insert(boundary.begin() + l, i);
 
    return l;
}
 
// Function to required count subarrays
vector<int> countingSubarray(vector<int> arr, int n)
{
     
    // Stores the indices of element
    unordered_map<int, int> index;
 
    for(int i = 0; i < n; i++)
        index[arr[i]] = i;
 
    vector<int> boundary = {-1, n};
    sort(arr.begin(), arr.end());
 
    // Initialize the output array
    vector<int> ans(n, 0);
 
    for(int num : arr)
    {
        int i = binaryInsert(boundary, index[num]);
 
        // Left boundary, till the
        // element is smallest
        int l = boundary[i] - boundary[i - 1] - 1;
 
        // Right boundary, till the
        // element is smallest
        int r = boundary[i + 1] - boundary[i] - 1;
 
        // Calculate the number of subarrays
        // based on its boundary
        int cnt = l + r + l * r + 1;
 
        // Adding cnt to the ans
        ans[index[num]] += cnt;
    }
    return ans;
}
 
// Driver Code
int main()
{
    int N = 5;
     
    // Given array arr[]
    vector<int> arr = { 3, 2, 4, 1, 5 };
     
    // Function call
    auto a = countingSubarray(arr, N);
     
    cout << "[";
    int n = a.size() - 1;
    for(int i = 0; i < n; i++)
        cout << a[i] << ", ";
         
    cout << a[n] << "]";
     
    return 0;
}
 
// This code is contributed by mohit kumar 29

Java

// Java program for the above approach
import java.util.*;
 
class GFG{
   
// Function to find the boundary of every
// element within which it is minimum
static int binaryInsert(ArrayList<Integer> boundary,
                        int i)
{
    int l = 0;
    int r = boundary.size() - 1;
   
    // Perform Binary Search
    while (l <= r)
    {
       
        // Find mid m
        int m = (l + r) / 2;
   
        // Update l
        if (boundary.get(m) < i)
            l = m + 1;
   
        // Update r
        else
            r = m - 1;
    }
   
    // Inserting the index
    boundary.add(l, i);
   
    return l;
}
   
// Function to required count subarrays
static int[] countingSubarray(int[] arr,
                              int n)
{
   
    // Stores the indices of element
    Map<Integer, Integer> index = new HashMap<>();
    for(int i = 0; i < n; i++)
        index.put(arr[i], i);
   
    ArrayList<Integer> boundary = new ArrayList<>();
    boundary.add(-1);
    boundary.add(n);
 
    Arrays.sort(arr);
 
    // Initialize the output array
    int[] ans = new int[n];
   
    for(int num : arr)
    {
        int i = binaryInsert(boundary,
                             index.get(num));
   
        // Left boundary, till the
        // element is smallest
        int l = boundary.get(i) -
                boundary.get(i - 1) - 1;
   
        // Right boundary, till the
        // element is smallest
        int r = boundary.get(i + 1) -
                boundary.get(i) - 1;
   
        // Calculate the number of subarrays
        // based on its boundary
        int cnt = l + r + l * r + 1;
   
        // Adding cnt to the ans
        ans[index.get(num)] += cnt;
    }
    return ans;
}
   
// Driver code
public static void main (String[] args)
{
    int N = 5;
       
    // Given array arr[]
    int[] arr = { 3, 2, 4, 1, 5 };
       
    // Function call
    int[] a = countingSubarray(arr, N);
       
    System.out.print("[");
    int n = a.length - 1;
    for(int i = 0; i < n; i++) 
        System.out.print(a[i] + ", ");
           
    System.out.print(a[n] + "]");
}
}
 
// This code is contributed by offbeat

Python3

# Python3 program for the above approach
 
# Function to find the boundary of every
# element within which it is minimum
def binaryInsert(boundary, i):
 
    l = 0
    r = len(boundary) - 1
 
    # Perform Binary Search
    while l <= r:
         
        # Find mid m
        m = (l + r) // 2
         
        # Update l
        if boundary[m] < i:
            l = m + 1
             
        # Update r
        else:
            r = m - 1
 
    # Inserting the index
    boundary.insert(l, i)
 
    return l
 
# Function to required count subarrays
def countingSubarray(arr, n):
 
    # Stores the indices of element
    index = {}
 
    for i in range(n):
        index[arr[i]] = i
 
    boundary = [-1, n]
    arr.sort()
 
    # Initialize the output array
    ans = [0 for i in range(n)]
 
    for num in arr:
        i = binaryInsert(boundary, index[num])
 
        # Left boundary, till the
        # element is smallest
        l = boundary[i] - boundary[i - 1] - 1
 
        # Right boundary, till the
        # element is smallest
        r = boundary[i + 1] - boundary[i] - 1
 
        # Calculate the number of subarrays
        # based on its boundary
        cnt = l + r + l * r + 1
 
        # Adding cnt to the ans
        ans[index[num]] += cnt
 
    return ans
 
 
# Driver Code
 
N = 5
 
# Given array arr[]
arr = [3, 2, 4, 1, 5]
 
# Function Call
print(countingSubarray(arr, N))

C#

// C# program for
// the above approach
using System;
using System.Collections;
using System.Collections.Generic;
class GFG{
    
// Function to find the
// boundary of every element
// within which it is minimum
static int binaryInsert(ArrayList boundary,
                        int i)
{
  int l = 0;
  int r = boundary.Count - 1;
 
  // Perform Binary Search
  while (l <= r)
  {
    // Find mid m
    int m = (l + r) / 2;
 
    // Update l
    if ((int)boundary[m] < i)
      l = m + 1;
 
    // Update r
    else
      r = m - 1;
  }
 
  // Inserting the index
  boundary.Insert(l, i);
 
  return l;
}
    
// Function to required count subarrays
static int[] countingSubarray(int[] arr,
                              int n)
{
  // Stores the indices of element
  Dictionary<int,
             int> index = new Dictionary<int,
                                         int>();
  for(int i = 0; i < n; i++)
    index[arr[i]] = i;
 
  ArrayList boundary = new ArrayList();
  boundary.Add(-1);
  boundary.Add(n);
  Array.Sort(arr);
 
  // Initialize the output array
  int[] ans = new int[n];
 
  foreach(int num in arr)
  {
    int i = binaryInsert(boundary,
                         index[num]);
 
    // Left boundary, till the
    // element is smallest
    int l = (int)boundary[i] -
            (int)boundary[i - 1] - 1;
 
    // Right boundary, till the
    // element is smallest
    int r = (int)boundary[i + 1] -
            (int)boundary[i] - 1;
 
    // Calculate the number of 
    // subarrays based on its boundary
    int cnt = l + r + l * r + 1;
 
    // Adding cnt to the ans
    ans[index[num]] += cnt;
  }
  return ans;
}
    
// Driver code
public static void Main(string[] args)
{
    int N = 5;
        
    // Given array arr[]
    int[] arr = {3, 2, 4, 1, 5};
        
    // Function call
    int[] a = countingSubarray(arr, N);
        
    Console.Write("[");
    int n = a.Length - 1;
    for(int i = 0; i < n; i++) 
        Console.Write(a[i] + ", ");
            
    Console.Write(a[n] + "]");
}
}
 
// This code is contributed by Rutvik_56

Output

[1, 4, 1, 8, 1]

Time Complexity: O(N log N)
Auxiliary Space: O(N)

Most efficient approach:

To optimize the above approach we can use a Stack Data Structure.

Idea is that, For each (1≤ i ≤ N) we will try to find index(R) of next smaller element right to it and index(L) of next smaller element left to it.
Now we have our boundary index(L,R) in which arr[i] is minimum so total number of subarrays for each i(0-base) will be (R-i)*(i-L) .

Below is the implementation of the idea:

C++14

// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to required count subarrays
vector<int> countingSubarray(vector<int> arr, int n)
{
    // For storing count of subarrays
    vector<int> a(n);
     
    // For finding next smaller
    // element left to a element
    // if there is no next smaller
    // element left to it than taking -1.
    vector<int> nsml(n, -1);
     
    // For finding next smaller
    // element right to a element
    // if there is no next smaller
    // element right to it than taking n.
    vector<int> nsmr(n, n);
 
    stack<int> st;
    for (int i = n - 1; i >= 0; i--)
    {
        while (!st.empty() && arr[st.top()] >= arr[i])
            st.pop();
        nsmr[i] = (!st.empty()) ? st.top() : n;
        st.push(i);
    }
    while (!st.empty())
        st.pop();
 
    for (int i = 0; i < n; i++)
    {
        while (!st.empty() && arr[st.top()] >= arr[i])
            st.pop();
        nsml[i] = (!st.empty()) ? st.top() : -1;
        st.push(i);
    }
 
    for (int i = 0; i < n; i++)
    {
        // Taking exact boundaries
        // in which arr[i] is
        // minimum
        nsml[i]++;
         
        // Similarly for right side
        nsmr[i]--;
        int r = nsmr[i] - i + 1;
        int l = i - nsml[i] + 1;
        a[i] = r * l;
    }
 
    return a;
}
 
// Driver Code
int main()
{
 
    int N = 5;
 
    // Given array arr[]
    vector<int> arr = { 3, 2, 4, 1, 5 };
 
    // Function call
    auto a = countingSubarray(arr, N);
 
    cout << "[";
    int n = a.size() - 1;
    for (int i = 0; i < n; i++)
        cout << a[i] << ", ";
 
    cout << a[n] << "]";
 
    return 0;
}

Java

// Java implementation of the above approach
import java.util.*;
public class gfg
{
  // Function to required count subarrays
  static int[] countingSubarray(int arr[], int n)
  {
 
    // For storing count of subarrays
    int a[] = new int[n];
 
    // For finding next smaller
    // element left to a element
    // if there is no next smaller
    // element left to it than taking -1.
    int nsml[] = new int[n];
    Arrays.fill(nsml, -1);
 
    // For finding next smaller
    // element right to a element
    // if there is no next smaller
    // element right to it than taking n.
    int nsmr[] = new int[n];
    Arrays.fill(nsmr, n);
 
    Stack<Integer> st = new Stack<Integer>();
    for(int i = n - 1; i >= 0; i--)
    {
      while (st.size() > 0 &&
             arr[(int)st.peek()] >= arr[i])
        st.pop();
 
      nsmr[i] = (st.size() > 0) ? (int)st.peek() : n;
      st.push(i);
    }
    while (st.size() > 0)
      st.pop();   
    for(int i = 0; i < n; i++)
    {
      while (st.size() > 0 &&
             arr[(int)st.peek()] >= arr[i])
        st.pop();
      nsml[i] = (st.size() > 0) ? (int)st.peek() : -1;
      st.push(i);
    }
    for(int i = 0; i < n; i++)
    {
 
      // Taking exact boundaries
      // in which arr[i] is
      // minimum
      nsml[i]++;
 
      // Similarly for right side
      nsmr[i]--;
      int r = nsmr[i] - i + 1;
      int l = i - nsml[i] + 1;
      a[i] = r * l;
    }
    return a;
  }
 
  // Driver code
  public static void main(String[] args)
  {
    int N = 5;
 
    // Given array arr[]
    int arr[] = { 3, 2, 4, 1, 5 };
 
    // Function call
    int a[] = countingSubarray(arr, N);
    System.out.print("[");
    int n = a.length - 1;
    for(int i = 0; i < n; i++)
      System.out.print(a[i] + ", ");
    System.out.print(a[n] + "]");
  }
}
 
// This code is contributed by divyesh072019.

Python3

# Python implementation of the above approach
 
# Function to required count subarrays
def countingSubarray(arr, n):
   
    # For storing count of subarrays
    a = [0 for i in range(n)]
     
    # For finding next smaller
    # element left to a element
    # if there is no next smaller
    # element left to it than taking -1.
    nsml = [-1 for i in range(n)]
     
    # For finding next smaller
    # element right to a element
    # if there is no next smaller
    # element right to it than taking n.
    nsmr = [n for i in range(n)]
    st = []
    for i in range(n - 1, -1, -1):
        while(len(st) > 0 and arr[st[-1]] >= arr[i]):
            del st[-1]
        if(len(st) > 0):
            nsmr[i] = st[-1]
        else:
            nsmr[i] = n
        st.append(i)
    while(len(st) > 0):
        del st[-1]
     
    for i in range(n):
        while(len(st) > 0 and arr[st[-1]] >= arr[i]):
            del st[-1]
         
        if(len(st) > 0):
            nsml[i] = st[-1]
        else:
            nsml[i] = -1
        st.append(i)
     
    for i in range(n):
         
        # Taking exact boundaries
        # in which arr[i] is
        # minimum
        nsml[i] += 1
         
        # Similarly for right side
        nsmr[i] -= 1
        r = nsmr[i] - i + 1;
        l = i - nsml[i] + 1;
        a[i] = r * l;  
    return a;
 
# Driver code
N = 5
 
# Given array arr[]
arr = [3, 2, 4, 1, 5 ]
 
# Function call
a = countingSubarray(arr, N)
print(a)
 
# This code is contributed by rag2127

C#

// C# implementation of the above approach
using System;
using System.Collections;
 
class GFG{
     
// Function to required count subarrays
static int[] countingSubarray(int[] arr, int n)
{
     
    // For storing count of subarrays
    int[] a = new int[n];
     
    // For finding next smaller
    // element left to a element
    // if there is no next smaller
    // element left to it than taking -1.
    int[] nsml = new int[n];
    Array.Fill(nsml, -1);
     
    // For finding next smaller
    // element right to a element
    // if there is no next smaller
    // element right to it than taking n.
    int[] nsmr = new int[n];
    Array.Fill(nsmr, n);
  
    Stack st = new Stack();
    for(int i = n - 1; i >= 0; i--)
    {
        while (st.Count > 0 &&
               arr[(int)st.Peek()] >= arr[i])
            st.Pop();
             
        nsmr[i] = (st.Count > 0) ? (int)st.Peek() : n;
        st.Push(i);
    }
    while (st.Count > 0)
        st.Pop();
  
    for(int i = 0; i < n; i++)
    {
        while (st.Count > 0 &&
               arr[(int)st.Peek()] >= arr[i])
            st.Pop();
             
        nsml[i] = (st.Count > 0) ? (int)st.Peek() : -1;
        st.Push(i);
    }
  
    for(int i = 0; i < n; i++)
    {
         
        // Taking exact boundaries
        // in which arr[i] is
        // minimum
        nsml[i]++;
          
        // Similarly for right side
        nsmr[i]--;
        int r = nsmr[i] - i + 1;
        int l = i - nsml[i] + 1;
        a[i] = r * l;
    }
    return a;
}
 
// Driver code
static void Main()
{
    int N = 5;
     
    // Given array arr[]
    int[] arr = { 3, 2, 4, 1, 5 };
     
    // Function call
    int[] a = countingSubarray(arr, N);
     
    Console.Write("[");
    int n = a.Length - 1;
     
    for(int i = 0; i < n; i++)
        Console.Write(a[i] + ", ");
     
    Console.Write(a[n] + "]");
}
}
 
// This code is contributed by divyeshrabadiya07

Javascript

<script>
    // Javascript implementation of the above approach
     
    // Function to required count subarrays
    function countingSubarray(arr, n)
    {
 
        // For storing count of subarrays
        let a = new Array(n);
 
        // For finding next smaller
        // element left to a element
        // if there is no next smaller
        // element left to it than taking -1.
        let nsml = new Array(n);
        nsml.fill(-1);
 
        // For finding next smaller
        // element right to a element
        // if there is no next smaller
        // element right to it than taking n.
        let nsmr = new Array(n);
        nsmr.fill(n);
 
        let st = [];
        for(let i = n - 1; i >= 0; i--)
        {
            while (st.length > 0 && arr[st[st.length-1]] >= arr[i])
                st.pop();
 
            nsmr[i] = (st.length > 0) ? st[st.length-1] : n;
            st.push(i);
        }
        while (st.length > 0)
            st.pop();
 
        for(let i = 0; i < n; i++)
        {
            while (st.length > 0 &&
                   arr[st[st.length-1]] >= arr[i])
                st.pop();
 
            nsml[i] = (st.length > 0) ? st[st.length-1] : -1;
            st.push(i);
        }
 
        for(let i = 0; i < n; i++)
        {
 
            // Taking exact boundaries
            // in which arr[i] is
            // minimum
            nsml[i]++;
 
            // Similarly for right side
            nsmr[i]--;
            let r = nsmr[i] - i + 1;
            let l = i - nsml[i] + 1;
            a[i] = r * l;
        }
        return a;
    }
     
    let N = 5;
      
    // Given array arr[]
    let arr = [ 3, 2, 4, 1, 5 ];
      
    // Function call
    let a = countingSubarray(arr, N);
      
    document.write("[");
    let n = a.length - 1;
      
    for(let i = 0; i < n; i++)
        document.write(a[i] + ", ");
      
    document.write(a[n] + "]");
     
    // This code is contributed by rameshtravel07.
</script>

Output

[1, 4, 1, 8, 1]

Time Complexity: O(N)
Auxiliary Space: O(N)

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