Problem

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:

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:

Solution: Hashtable

Count how many people at each age i.

Time complexity: O(n)

Space complexity: O(120)

C++

// Author: Huahua

// Running time: 39 ms

class Solution {

public:

int numFriendRequests(vector<int>& ages) {

const int kMaxAge = 120;

vector<int> counts(kMaxAge + 1, 0);

for (const int age : ages)

++counts[age];

int ans = 0;

for (int A = 1; A <= kMaxAge; ++A) {

for (int B = 0.5 * A + 7 + 1; B <= A; ++B)

ans += counts[A] * (counts[B] - (A == B));

}

return ans;

}

};

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