# ---
# title: 825. Friends Of Appropriate Ages
# id: problem825
# author: Tian Jun
# date: 2020-10-31
# difficulty: Medium
# categories: Array
# link: <https://leetcode.com/problems/friends-of-appropriate-ages/description/>
# hidden: true
# ---
# 
# Some people will make friend requests. The list of their ages is given and
# `ages[i]` is the age of the ith person.
# 
# Person A will NOT friend request person B (B != A) if any of the following
# conditions are true:
# 
#   * `age[B] <= 0.5 * age[A] + 7`
#   * `age[B] > age[A]`
#   * `age[B] > 100 && age[A] < 100`
# 
# Otherwise, A will friend request B.
# 
# Note that if A requests B, B does not necessarily request A.  Also, people
# will not friend request themselves.
# 
# How many total friend requests are made?
# 
# **Example 1:**
# 
#     
#     
#     Input: [16,16]
#     Output: 2
#     Explanation: 2 people friend request each other.
#     
# 
# **Example 2:**
# 
#     
#     
#     Input: [16,17,18]
#     Output: 2
#     Explanation: Friend requests are made 17 -> 16, 18 -> 17.
# 
# **Example 3:**
# 
#     
#     
#     Input: [20,30,100,110,120]
#     Output: 3
#     Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.
#     
# 
# 
# 
# Notes:
# 
#   * `1 <= ages.length <= 20000`.
#   * `1 <= ages[i] <= 120`.
# 
# 
## @lc code=start
using LeetCode

## add your code here:
## @lc code=end
