Number of valid indices in the permutation of first N natural numbers

Given a permutation P of first N natural numbers. The task is to find the number of i’s such that P_i ≤ P_j for all 1 ≤ j ≤ i in the permutation of first N natural numbers.
Examples:

Input: arr[] = {4, 2, 5, 1, 3} 
Output: 3 
0, 1 and 3 are such indices.
Input: arr[] = {4, 3, 2, 1} 
Output: 4 

Approach: For i = 1, …, N, define M_i = min{ P_j, 1 ≤ j ≤ i}. Now, when i(1 ≤ i ≤ N) is fixed, M_i = P_i holds if and only if for all j(1 ≤ j ≤ i), P_i ≤ P_j holds. Therefore, it is enough to calculate all the M_i. These can be calculated in the increasing order of i, so the problem could be solved in a total of O(N) time.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to return the number of i's
// such that Pi <= Pj for all 1 <= j <= i
// in the permutation of first N natural numbers
int min_index(int p[], int n)
{

    // To store the count of such indices
    int ans = 0;

    // Store the mini value
    int mini = INT_MAX;

    // For all the elements
    for (int i = 0; i < n; i++) {
        if (p[i] <= mini)
            mini = p[i];

        if (mini == p[i])
            ans++;
    }

    // Return the required answer
    return ans;
}

// Driver code
int main()
{
    int P[] = { 4, 2, 5, 1, 3 };
    int n = sizeof(P) / sizeof(int);

    cout << min_index(P, n);

    return 0;
}

Java

// Java implementation of the approach
class GFG
{

    static int INT_MAX = Integer.MAX_VALUE;

    // Function to return the number of i's
    // such that Pi <= Pj for all 1 <= j <= i
    // in the permutation of first N natural numbers
    static int min_index(int p[], int n)
    {

        // To store the count of such indices
        int ans = 0;

        // Store the mini value
        int mini = INT_MAX;

        // For all the elements
        for (int i = 0; i < n; i++)
        {
            if (p[i] <= mini)
                mini = p[i];

            if (mini == p[i])
                ans++;
        }

        // Return the required answer
        return ans;
    }

    // Driver code
    public static void main (String[] args)
    {
        int P[] = { 4, 2, 5, 1, 3 };
        int n = P.length;

        System.out.println(min_index(P, n));
    }
}

// This code is contributed by AnkitRai01

Python3

# Python3 implementation of the approach
import sys

INT_MAX = sys.maxsize

# Function to return the number of i's
# such that Pi <= Pj for all 1 <= j <= i
# in the permutation of first N natural numbers
def min_index(p, n) :

    # To store the count of such indices
    ans = 0;

    # Store the mini value
    mini = INT_MAX;

    # For all the elements
    for i in range(n) :
        if (p[i] <= mini) :
            mini = p[i];

        if (mini == p[i]) :
            ans += 1;

    # Return the required answer
    return ans;

# Driver code
if __name__ == "__main__" :

    P = [ 4, 2, 5, 1, 3 ];
    n = len(P);
    print(min_index(P, n));

# This code is contributed by AnkitRai01

C#

// C# implementation of the approach
using System;

class GFG
{

    static int INT_MAX = int.MaxValue;

    // Function to return the number of i's
    // such that Pi <= Pj for all 1 <= j <= i
    // in the permutation of first N natural numbers
    static int min_index(int []p, int n)
    {

        // To store the count of such indices
        int ans = 0;

        // Store the mini value
        int mini = INT_MAX;

        // For all the elements
        for (int i = 0; i < n; i++)
        {
            if (p[i] <= mini)
                mini = p[i];

            if (mini == p[i])
                ans++;
        }

        // Return the required answer
        return ans;
    }

    // Driver code
    public static void Main ()
    {
        int []P = { 4, 2, 5, 1, 3 };
        int n = P.Length;

        Console.WriteLine(min_index(P, n));
    }
}

// This code is contributed by AnkitRai01

Javascript

<script>

// JavaScript implementation of the approach

// Function to return the number of i's
// such that Pi <= Pj for all 1 <= j <= i
// in the permutation of first N natural numbers
function min_index(p, n)
{

    // To store the count of such indices
    let ans = 0;

    // Store the mini value
    let mini = Number.MAX_SAFE_INTEGER;

    // For all the elements
    for (let i = 0; i < n; i++) {
        if (p[i] <= mini)
            mini = p[i];

        if (mini == p[i])
            ans++;
    }

    // Return the required answer
    return ans;
}

// Driver code

    let P = [ 4, 2, 5, 1, 3 ];
    let n = P.length;

    document.write(min_index(P, n));

// This code is contributed by Manoj.

</script>

Output:

3

Time Complexity: O(n)

Auxiliary Space: O(1)