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Count of Subarrays whose first element is the minimum

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  • Difficulty Level : Medium
  • Last Updated : 01 Jul, 2022
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Given an array arr[] of size N, the task is to find the number of subarrays whose first element is not greater than other elements of the subarray.

Examples:

Input: arr = {1, 2, 1}
Output: 5
Explanation: All subarray are: {1}, {1, 2}, {1, 2, 1}, {2}, {2, 1}, {1}
From above subarray the following meets the condition: {1}, {1, 2}, {1, 2, 1}, {2}, {1}

Input: arr[] = {1, 3, 5, 2}
Output: 8
Explanation: We have the following subarrays which meet the condition:
{1}, {1, 3}, {1, 3, 5}, {1, 3, 5, 2}, {3}, {3, 5}, {5}, {2}

 

Naive Approach: The naive approach is to run a nested loop and find all the subarrays with first element not bigger than the other elements in the subarray.

Below is the implementation of the above approach :

C++




// C++ code to implement the approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find all subarrays with first
// element not bigger than other elements
int countSubarrays(vector<int>& arr)
{
    int n = arr.size();
 
    // Cnt is initialised to n because
    // atleast n subarrays will be there
    // which is single element itself
    int cnt = n;
 
    // Two loops to find the
    // ending of each subarrays
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
 
            // Minimum element from
            // [start+1, end] of each subarray
            int mini_ele = *min_element(begin(arr) + i + 1,
                                        begin(arr) + j + 1);
 
            // Checking if minimum of
            // elements from [start+1, end] is
            // not smaller than start element
            // updating the count of subarrays
            if (mini_ele >= arr[i])
                cnt++;
        }
    }
    return cnt;
}
 
// Driver Code
int main()
{
    vector<int> arr = { 1, 3, 5, 2 };
 
    // Function call
    cout << countSubarrays(arr) << "\n";
    return 0;
}


Java




// Java code to implement the approach
 
import java.io.*;
import java.util.*;
  
class GFG
{
    // Function to find the minimum element in a given range
    public static int min_element(int arr[], int left, int right)
    {
        int x = Integer.MAX_VALUE;
        for(int i=left;i<right;i++)
        {
            x=Math.min(x,arr[i]);
        }
        return x;
    }
     
    // Function to find all subarrays with first
    // element not bigger than other elements
    public static int countSubarrays(int arr[])
    {
        int n = arr.length;
         
        // Cnt is initialised to n because
        // atleast n subarrays will be there
        // which is single element itself
        int cnt = n;
         
        // Two loops to find the
        // ending of each subarrays
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                // Minimum element from
                // [start+1, end] of each subarray
                int mini_ele = min_element(arr,i+1,j+1);
                 
                // Checking if minimum of
                // elements from [start+1, end] is
                // not smaller than start element
                // updating the count of subarrays
                if (mini_ele >= arr[i])
                cnt++;
            }
        }
         
        return cnt;
    }
     
    // Driver Code
    public static void main(String[] args)
    {
        int arr[] = {1, 3, 5, 2};
         
        // Function call
        System.out.println(countSubarrays(arr) );
    }
}
 
// This code is contributed by Aditya Patil


Python3




# Python3 code to implement the approach
 
# Function to find all subarrays with first
# element not bigger than other elements
 
# Function to find the minimum element in a given range
import sys
 
def min_element(arr, left, right):
    x = sys.maxsize
 
    for i in range(left, right):
        x = min(arr[i], x)
    return x
 
def countSubarrays(arr):
    n = len(arr)
 
    # Cnt is initialised to n because
    # atleast n subarrays will be there
    # which is single element itself
    cnt = n
 
    # Two loops to find the
    # ending of each subarrays
    for i in range(n):
        for j in range(i+1,n):
 
            # Minimum element from
            # [start+1, end] of each subarray
            mini_ele = min_element(arr ,i + 1, j + 1)
 
            # Checking if minimum of
            # elements from [start+1, end] is
            # not smaller than start element
            # updating the count of subarrays
            if (mini_ele >= arr[i]):
                cnt += 1
     
    return cnt
 
# Driver Code
arr = [ 1, 3, 5, 2 ]
 
# Function call
print(countSubarrays(arr))
 
# This code is contributed by shinjanpatra


C#




// C# code to implement the approach
using System;
using System.Collections.Generic;
 
public class GFG
{
   
  // Function to find the minimum element in a given range
  public static int min_element(int []arr, int left, int right)
  {
    int x = int.MaxValue;
    for(int i=left;i<right;i++)
    {
      x=Math.Min(x,arr[i]);
    }
    return x;
  }
 
  // Function to find all subarrays with first
  // element not bigger than other elements
  public static int countSubarrays(int []arr)
  {
    int n = arr.Length;
 
    // Cnt is initialised to n because
    // atleast n subarrays will be there
    // which is single element itself
    int cnt = n;
 
    // Two loops to find the
    // ending of each subarrays
    for(int i=0;i<n;i++)
    {
      for(int j=i+1;j<n;j++)
      {
        // Minimum element from
        // [start+1, end] of each subarray
        int mini_ele = min_element(arr,i+1,j+1);
 
        // Checking if minimum of
        // elements from [start+1, end] is
        // not smaller than start element
        // updating the count of subarrays
        if (mini_ele >= arr[i])
          cnt++;
      }
    }
 
    return cnt;
  }
 
  // Driver Code
  public static void Main(String[] args)
  {
    int []arr = {1, 3, 5, 2};
 
    // Function call
    Console.WriteLine(countSubarrays(arr) );
  }
}
 
// This code contributed by shikhasingrajput


Javascript




<script>
 
// JavaScript code to implement the approach
 
// Function to find all subarrays with first
// element not bigger than other elements
 
// Function to find the minimum element in a given range
function min_element(arr,left,right){
    let x = Number.MAX_VALUE
    for(let i=left;i<right;i++){
        x = Math.min(arr[i],x)
    }
    return x
}
 
function countSubarrays(arr)
{
    let n = arr.length
 
    // Cnt is initialised to n because
    // atleast n subarrays will be there
    // which is single element itself
    let cnt = n
 
    // Two loops to find the
    // ending of each subarrays
    for (let i = 0; i < n; i++) {
        for (let j = i + 1; j < n; j++) {
 
            // Minimum element from
            // [start+1, end] of each subarray
            let mini_ele = min_element(arr ,i + 1, j + 1);
 
            // Checking if minimum of
            // elements from [start+1, end] is
            // not smaller than start element
            // updating the count of subarrays
            if (mini_ele >= arr[i])
                cnt++;
        }
    }
    return cnt;
}
 
// Driver Code
 
let arr = [ 1, 3, 5, 2 ];
 
// Function call
document.write(countSubarrays(arr),"</br>");
 
// This code is contributed by shinjanpatra
 
</script>


 
 

Output

8

 

Time Complexity: O(N3)
Auxiliary Space: O(1)

 

Efficient Approach:  The efficient approach is based on the concept of finding the next smaller element on the right of an element. To implement this concept stack is used. Follow the steps mentioned below:

 

  • We use a monotonic stack to get the index of the next smaller of each element on the right because we want the subarray with first element as the minimum element.
  • The total subarray’s in the range [i, j] having i as the starting index is (j – i).
  • Compute the next smaller index for every index i, add (j-i) for each of them and keep updating the total count of valid subarrays.

 

Below is the implementation of the above approach.

 

C++




// C++ code to implement the approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find all subarrays with first
// element not bigger than other elements
int countSubarrays(vector<int>& arr)
{
    // Taking stack for find next smaller
    // element to the right
    stack<int> s;
    int ans = 0;
    int n = arr.size();
 
    // Looping from right side because we next
    // smallest to the right
    for (int i = n - 1; i >= 0; i--) {
        while (!s.empty() and arr[s.top()] >= arr[i])
            s.pop();
        // The index of next smaller element
        // starting from i'th index
        int last = (s.empty() ? n : s.top());
 
        // Adding the number of subarray which
        // can be formed in the range [i, last]
        ans += (last - i);
        s.push(i);
    }
    return ans;
}
 
// Driver Code
int main()
{
    vector<int> arr = { 1, 3, 5, 2 };
 
    // Function call
    cout << countSubarrays(arr) << "\n";
    return 0;
}


Java




// JAVA code to implement the approach
import java.util.*;
class GFG {
 
  // Function to find all subarrays with first
  // element not bigger than other elements
  public static int countSubarrays(int arr[])
  {
 
    // Taking stack for find next smaller
    // element to the right
    Stack<Integer> s = new Stack<Integer>();
    int ans = 0;
    int n = arr.length;
 
    // Looping from right side because we next
    // smallest to the right
    for (int i = n - 1; i >= 0; i--) {
      while (s.empty() == false
             && arr[s.peek()] >= arr[i])
        s.pop();
      // The index of next smaller element
      // starting from i'th index
      int last = ((s.empty() == true) ? n : s.peek());
 
      // Adding the number of subarray which
      // can be formed in the range [i, last]
      ans += (last - i);
      s.push(i);
    }
    return ans;
  }
 
  // Driver Code
  public static void main(String[] args)
  {
    int arr[] = new int[] { 1, 3, 5, 2 };
 
    // Function call
    System.out.println(countSubarrays(arr));
  }
}
 
// This code is contributed by Taranpreet


Python3




# Python 3 code to implement the approach
 
# Function to find all subarrays with first
# element not bigger than other elements
def countSubarrays(arr):
 
    # Taking stack for find next smaller
    # element to the right
    s = []
    ans = 0
    n = len(arr)
 
    # Looping from right side because we next
    # smallest to the right
    for i in range(n - 1, -1, -1):
        while (len(s) != 0 and arr[s[-1]] >= arr[i]):
            s.pop()
             
        # The index of next smaller element
        # starting from i'th index
        if (len(s) == 0):
            last = n
        else:
            last = s[-1]
 
        # Adding the number of subarray which
        # can be formed in the range [i, last]
        ans += (last - i)
        s.append(i)
 
    return ans
 
# Driver Code
if __name__ == "__main__":
 
    arr = [1, 3, 5, 2]
 
    # Function call
    print(countSubarrays(arr))
 
    # This code is contributed by ukasp.


C#




// C# program to implement above approach
using System;
using System.Collections.Generic;
 
class GFG
{
  // Function to find all subarrays with first
  // element not bigger than other elements
  public static int countSubarrays(List<int> arr)
  {
    // Taking stack for find next smaller
    // element to the right
    Stack<int> s = new Stack<int>();
    int ans = 0;
    int n = arr.Count;
 
    // Looping from right side because we next
    // smallest to the right
    for (int i = n - 1 ; i >= 0 ; i--) {
      while (s.Count > 0 && arr[s.Peek()] >= arr[i]){
        s.Pop();
      }
      // The index of next smaller element
      // starting from i'th index
      int last = (s.Count == 0 ? n : s.Peek());
 
      // Adding the number of subarray which
      // can be formed in the range [i, last]
      ans += (last - i);
      s.Push(i);
    }
    return ans;
  }
 
  // Driver Code
  public static void Main(string[] args){
 
    List<int> arr = new List<int>{
      1, 3, 5, 2
      };
 
    // Function call
    Console.Write(countSubarrays(arr));
  }
}
 
// This code is contributed by subhamgoyal2014.


Javascript




<script>
    // JavaScript code to implement the approach
 
    // Function to find all subarrays with first
    // element not bigger than other elements
    const countSubarrays = (arr) => {
     
        // Taking stack for find next smaller
        // element to the right
        let s = [];
        let ans = 0;
        let n = arr.length;
 
        // Looping from right side because we next
        // smallest to the right
        for (let i = n - 1; i >= 0; i--)
        {
            while (s.length != 0 && arr[s[s.length - 1]] >= arr[i])
                s.pop();
                 
            // The index of next smaller element
            // starting from i'th index
            let last = (s.length == 0 ? n : s[s.length - 1]);
 
            // Adding the number of subarray which
            // can be formed in the range [i, last]
            ans += (last - i);
            s.push(i);
        }
        return ans;
    }
 
    // Driver Code
 
    let arr = [1, 3, 5, 2];
 
    // Function call
    document.write(countSubarrays(arr));
 
    // This code is contributed by rakeshsahni
 
</script>


 
 

Output

8

 

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

 


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