## C# || How To Find Friends Of Appropriate Ages Using C# The following is a module with functions which demonstrates how to find friends of appropriate ages using C#.

1. Num Friend Requests – Problem Statement

There are n persons on a social media website. You are given an integer array ages where ages[i] is the age of the ith person.

A Person x will not send a friend request to a person y (x != y) if any of the following conditions is true:

• age[y] <= 0.5 * age[x] + 7
• age[y] > age[x]
• age[y] > 100 && age[x] < 100

Otherwise, x will send a friend request to y.

Note that if x sends a request to y, y will not necessarily send a request to x. Also, a person will not send a friend request to themself.

Return the total number of friend requests made.

Example 1:

``` Input: ages = [16,16] Output: 2 Explanation: 2 people friend request each other. ```

Example 2:

``` Input: ages = [16,17,18] Output: 2 Explanation: Friend requests are made 17 -> 16, 18 -> 17. ```

Example 3:

``` Input: ages = [20,30,100,110,120] Output: 3 Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100. ```

2. Num Friend Requests – Solution

The following is a solution which demonstrates how to find friends of appropriate ages.

In this solution, we sort the ages and use a map that keeps track of the ages, and the total friend request count that age can recieve.

Binary search is used to get the age range in the array that satisfies the friend request conditions for a given age.

``` 2. Num Friend Requests - Solution C# // ============================================================================ // Author: Kenneth Perkins // Date: Nov 5, 2021 // Taken From: http://programmingnotes.org/ // File: Solution.cs // Description: Demonstrates how to find friends of appropriate ages // ============================================================================ public class Solution { public int NumFriendRequests(int[] ages) { var result = 0; // Sort ages Array.Sort(ages); // Keep track of results calculated var seen = new Dictionary<int, int>(); // Iterate array backwards for (int index = ages.Length - 1; index >= 0; --index) { // Check if we already calculated result for age if (!seen.ContainsKey(ages[index])) { // Binary search var lo = 0; var hi = index; while (lo < hi) { var mid = lo + (hi - lo) / 2; // Check if friends can be made in this range if (!CanFriend(ages[index], ages[mid])) { lo = mid + 1; } else { hi = mid; } } // Get distance between index and hi. This is the friend count for the age seen[ages[index]] = index - hi; } // Update result count result += seen[ages[index]]; } return result; } private bool CanFriend(int x, int y) { return !((y > x) || (y > 100 && x < 100) || (y <= 0.5 * x + 7)); } }// http://programmingnotes.org/ 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748 // ============================================================================//    Author: Kenneth Perkins//    Date:   Nov 5, 2021//    Taken From: http://programmingnotes.org///    File:  Solution.cs//    Description: Demonstrates how to find friends of appropriate ages// ============================================================================public class Solution {    public int NumFriendRequests(int[] ages) {        var result = 0;         // Sort ages        Array.Sort(ages);         // Keep track of results calculated        var seen = new Dictionary<int, int>();         // Iterate array backwards        for (int index = ages.Length - 1; index >= 0; --index) {            // Check if we already calculated result for age            if (!seen.ContainsKey(ages[index])) {                // Binary search                var lo = 0;                var hi = index;                while (lo < hi) {                    var mid = lo + (hi - lo) / 2;                     // Check if friends can be made in this range                    if (!CanFriend(ages[index], ages[mid])) {                        lo = mid + 1;                    } else {                        hi = mid;                    }                }                // Get distance between index and hi. This is the friend count for the age                seen[ages[index]] = index - hi;            }            // Update result count            result += seen[ages[index]];        }         return result;    }     private bool CanFriend(int x, int y) {        return !((y > x) || (y > 100 && x < 100) || (y <= 0.5 * x + 7));    }}// http://programmingnotes.org/ ```

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output for the example cases:

``` 2 2 3 ```