How hard is LeetCode 825. Friends Of Appropriate Ages?

LeetCode 825. Friends Of Appropriate Ages is rated Medium on LeetCode.

What is the time complexity of LeetCode 825. Friends Of Appropriate Ages?

The Python solution on this page runs in O(n + m^2), where n is the length of the array \textit{ages}, and m is the maximum age, which is 121 in this problem.

What is the space complexity of LeetCode 825. Friends Of Appropriate Ages?

The Python solution on this page uses O(1) auxiliary space.

What topics does LeetCode 825. Friends Of Appropriate Ages cover?

LeetCode 825. Friends Of Appropriate Ages is tagged Array, Two Pointers, Binary Search and Sorting on LeetCode.

Friends Of Appropriate Ages — LeetCode 825 Python Solution

MediumArrayTwo PointersBinary SearchSorting

Problem: #825
Pattern: Two Pointers
Reading time: 2 min
Source: leetcode.com

The problem

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.

Example

Input: ages = [16,16]
Output: 2
Explanation: 2 people friend request each other.

Python solution

Python

class Solution:
    def numFriendRequests(self, ages: List[int]) -> int:
        cnt = [0] * 121
        for x in ages:
            cnt[x] += 1
        ans = 0
        for ax, x in enumerate(cnt):
            for ay, y in enumerate(cnt):
                if not (ay <= 0.5 * ax + 7 or ay > ax or (ay > 100 and ax < 100)):
                    ans += x * (y - int(ax == ay))
        return ans

Complexity

Measure	Complexity
Time	O(n + m^2), where n is the length of the array \textit{ages}, and m is the maximum age, which is 121 in this problem
Space	O(1) auxiliary

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 825. Friends Of Appropriate Ages is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 825. Friends Of Appropriate Ages?: LeetCode 825. Friends Of Appropriate Ages is rated Medium on LeetCode.
What is the time complexity of LeetCode 825. Friends Of Appropriate Ages?: The Python solution on this page runs in O(n + m^2), where n is the length of the array \textit{ages}, and m is the maximum age, which is 121 in this problem.
What is the space complexity of LeetCode 825. Friends Of Appropriate Ages?: The Python solution on this page uses O(1) auxiliary space.
What topics does LeetCode 825. Friends Of Appropriate Ages cover?: LeetCode 825. Friends Of Appropriate Ages is tagged Array, Two Pointers, Binary Search and Sorting on LeetCode.