Practice/Meta/Leetcode 825. Friends Of Appropriate Ages

Leetcode 825. Friends Of Appropriate Ages

CodingOptional

Problem

You are building a social network feature that determines whether person A can send a friend request to person B based on their ages. A friend request from person A (with age X) to person B (with age Y) will NOT be sent if any of the following conditions are true:

1. Y ≤ 0.5 * X + 7
2. Y > X
3. Y > 100 AND X < 100

Given an array of integers representing the ages of people in the network, return the total number of valid friend requests that can be made.

Note that a person cannot send a friend request to themselves, and if person A can send a request to person B, it does not automatically mean person B can send a request to person A.

Requirements

1. Calculate the total number of valid directed friend request pairs (A → B)
2. A person cannot send a request to themselves
3. Apply all three age-based filtering rules correctly
4. Handle duplicate ages in the input array (each person is distinct)

Constraints

• 1 ≤ ages.length ≤ 20,000
• 1 ≤ ages[i] ≤ 120
• Each element represents a distinct person, even if ages repeat
• Return the count of valid ordered pairs

Examples

Example 1:

` Input: ages = [16, 16, 17] Output: 4 Explanation: Valid requests:

• Person 0 (age 16) → Person 1 (age 16): Valid
• Person 1 (age 16) → Person 0 (age 16): Valid
• Person 2 (age 17) → Person 0 (age 16): Valid
• Person 2 (age 17) → Person 1 (age 16): Valid Total: 4 requests `

Example 2:

` Input: ages = [20, 30, 100, 110, 120] Output: 3 Explanation: Valid requests exist between certain age pairs:

• Person 1 (age 30) → Person 0 (age 20): Valid
• Person 3 (age 110) → Person 2 (age 100): Valid
• Person 4 (age 120) → Person 2 (age 100): Valid `

Example 3:

Input: ages = [8, 8, 8, 8] Output: 0 Explanation: For age 8, condition 1 fails: 8 ≤ 0.5 * 8 + 7 = 11, so no valid requests possible