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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

• Java
• C++
• Python
• Go

• 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
  }
  

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