Formatted question description: https://leetcode.ca/all/825.html

825. Friends Of Appropriate Ages (Medium)

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.

Companies:
Facebook

Related Topics:
Array

Solution 1.

// OJ: https://leetcode.com/problems/friends-of-appropriate-ages/
// Time: O(N)
// Space: O(1)
class Solution {
private:
    bool request(int A, int B) {
        return !(B <= .5 * A + 7 || B > A);
    }
public:
    int numFriendRequests(vector<int>& ages) {
        int ans = 0;
        vector<int> cnts(120, 0);
        for (int age : ages) cnts[age - 1]++;
        for (int i = 0; i < 120; ++i) {
            if (!cnts[i]) continue;
            for (int j = 0; j < 120; ++j) {
                if (!cnts[j] || !request(i + 1, j + 1)) continue;
                ans += cnts[i] * cnts[j];
                if (i == j) ans -= cnts[i];
            }
        }
        return ans;
    }
};

Java

  • class Solution {
        public int numFriendRequests(int[] ages) {
            Arrays.sort(ages);
            Map<Integer, Integer> ageCountMap = new HashMap<Integer, Integer>();
            for (int age : ages) {
                int count = ageCountMap.getOrDefault(age, 0) + 1;
                ageCountMap.put(age, count);
            }
            int friendRequests = 0;
            int length = ages.length;
            for (int i = 0; i < length; i++) {
                if (i > 0 && ages[i] == ages[i - 1])
                    continue;
                int age = ages[i];
                int minAge = age / 2 + 8;
                if (minAge > age)
                    continue;
                int minIndex = binarySearch(ages, minAge);
                int count = ageCountMap.get(age);
                friendRequests += count * (i - minIndex);
                friendRequests += count * (count - 1);
            }
            return friendRequests;
        }
    
        public int binarySearch(int[] ages, int minAge) {
            int low = 0, high = ages.length - 1;
            while (low < high) {
                int mid = (high - low) / 2 + low;
                int age = ages[mid];
                if (age < minAge)
                    low = mid + 1;
                else
                    high = mid;
            }
            return low;
        }
    }
    
  • // OJ: https://leetcode.com/problems/friends-of-appropriate-ages/
    // Time: O(N)
    // Space: O(1)
    class Solution {
    private:
        bool request(int A, int B) {
            return !(B <= .5 * A + 7 || B > A);
        }
    public:
        int numFriendRequests(vector<int>& ages) {
            int ans = 0;
            vector<int> cnts(120, 0);
            for (int age : ages) cnts[age - 1]++;
            for (int i = 0; i < 120; ++i) {
                if (!cnts[i]) continue;
                for (int j = 0; j < 120; ++j) {
                    if (!cnts[j] || !request(i + 1, j + 1)) continue;
                    ans += cnts[i] * cnts[j];
                    if (i == j) ans -= cnts[i];
                }
            }
            return ans;
        }
    };
    
  • class Solution(object):
        def numFriendRequests(self, ages):
            """
            :type ages: List[int]
            :rtype: int
            """
            count = collections.Counter(ages)
            ages = sorted(count.keys())
            N = len(ages)
            res = 0
            for A in ages:
                for B in range(int(0.5 * A) + 7 + 1, A + 1):
                    res += count[A] * (count[B] - int(A == B))
            return res
    

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