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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

  • 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 } 
  • function numFriendRequests(ages: number[]): number { const m = 121; const cnt = Array(m).fill(0); for (const x of ages) { cnt[x]++; } let ans = 0; for (let ax = 0; ax < m; ax++) { for (let ay = 0; ay < m; ay++) { if (ay <= 0.5 * ax + 7 || ay > ax || (ay > 100 && ax < 100)) { continue; } ans += cnt[ax] * (cnt[ay] - (ax === ay ? 1 : 0)); } } return ans; }

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