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Number of valid indices in the permutation of first N natural numbers
• Difficulty Level : Easy
• Last Updated : 12 Apr, 2021

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

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

Approach: For i = 1, …, N, define Mi = min{ Pj, 1 ≤ j ≤ i}. Now, when i(1 ≤ i ≤ N) is fixed, Mi = Pi holds if and only if for all j(1 ≤ j ≤ i), Pi ≤ Pj holds. Therefore, it is enough to calculate all the Mi. 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 ``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

 ``
Output:
`3`

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