LeetCode [825] Friends Of Appropriate Ages

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: 
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.

Notes:

• 1 <= ages.length <= 20000.
• 1 <= ages[i] <= 120.

class Solution { public: int numFriendRequests(vector& ages) { int res = 0; int numInAge[121] = {0}; int sumInAge[121] = {0}; for(int i : ages) numInAge[i] ++; for(int i = 1; i <= 120; ++i) sumInAge[i] = numInAge[i] + sumInAge[i - 1]; for(int i = 15; i <= 120; ++i) { if(numInAge[i] == 0) continue; int count = sumInAge[i] - sumInAge[i / 2 + 7]; res += count * numInAge[i] - numInAge[i]; //people will not friend request themselves, so - numInAge[i] } return res; } };