825. Friends Of Appropriate Ages

Description

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.


Constraints:

• n == ages.length
• 1 <= n <= 2 * 104
• 1 <= ages[i] <= 120

Solutions

• class Solution {
public int numFriendRequests(int[] ages) {
int[] counter = new int[121];
for (int age : ages) {
++counter[age];
}
int ans = 0;
for (int i = 1; i < 121; ++i) {
int n1 = counter[i];
for (int j = 1; j < 121; ++j) {
int n2 = counter[j];
if (!(j <= 0.5 * i + 7 || j > i || (j > 100 && i < 100))) {
ans += n1 * n2;
if (i == j) {
ans -= n2;
}
}
}
}
return ans;
}
}

• class Solution {
public:
int numFriendRequests(vector<int>& ages) {
vector<int> counter(121);
for (int age : ages) ++counter[age];
int ans = 0;
for (int i = 1; i < 121; ++i) {
int n1 = counter[i];
for (int j = 1; j < 121; ++j) {
int n2 = counter[j];
if (!(j <= 0.5 * i + 7 || j > i || (j > 100 && i < 100))) {
ans += n1 * n2;
if (i == j) ans -= n2;
}
}
}
return ans;
}
};

• class Solution:
def numFriendRequests(self, ages: List[int]) -> int:
counter = Counter(ages)
ans = 0
for i in range(1, 121):
n1 = counter[i]
for j in range(1, 121):
n2 = counter[j]
if not (j <= 0.5 * i + 7 or j > i or (j > 100 and i < 100)):
ans += n1 * n2
if i == j:
ans -= n2
return ans


• func numFriendRequests(ages []int) int {
counter := make([]int, 121)
for _, age := range ages {
counter[age]++
}
ans := 0
for i := 1; i < 121; i++ {
n1 := counter[i]
for j := 1; j < 121; j++ {
n2 := counter[j]
if !(j <= i/2+7 || j > i || (j > 100 && i < 100)) {
ans += n1 * n2
if i == j {
ans -= n2
}
}
}
}
return ans
}