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

Solution 1

Python Code
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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

Java Code
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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;
    }
}

C++ Code
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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;
    }
};

Go Code
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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
}