LeetCode 825: Friends Of Appropriate Ages Solution

Master LeetCode problem 825 (Friends Of Appropriate Ages), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

825. Friends Of Appropriate Ages

Medium

Array Two Pointers Binary Search Sorting

Problem Explanation

Explanation

To solve this problem, we can iterate through each pair of ages and check if the conditions for sending a friend request are met. We need to keep track of the count of valid friend requests made. We can optimize the process by using a frequency array to store the count of each age, as we only need to consider ages within the given constraints.

Create a frequency array to store the count of each age in the input array.
Iterate through each possible pair of ages (x, y) and check if the conditions for sending a friend request are satisfied.
If the conditions are met, increment the count of friend requests made.
Return the total count of friend requests made.

Time Complexity: O(n + A), where n is the number of ages and A is the maximum age (120 in this case). Space Complexity: O(A) for the frequency array.

Solution Code

class Solution {
    public int numFriendRequests(int[] ages) {
        int[] freq = new int[121];
        for (int age : ages) {
            freq[age]++;
        }
        
        int count = 0;
        for (int i = 0; i <= 120; i++) {
            for (int j = 0; j <= 120; j++) {
                if (!(j <= 0.5 * i + 7 || j > i || (j > 100 && i < 100))) {
                    count += freq[i] * freq[j];
                    if (i == j) {
                        count -= freq[i];
                    }
                }
            }
        }
        
        return count;
    }
}

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