Mutual Follow Pairs

Problem

A social app stores who follows whom as a list of pairs [a, b], meaning person a follows person b, with people numbered 0 to n - 1. The list may contain the same pair more than once, and nobody follows themselves. Return how many unordered pairs of people follow each other.

Examples

Input:  n = 4, follows = [[0, 1], [1, 0], [1, 2], [2, 3], [3, 2], [0, 2]]
Output: 2
Why:    0 and 1, and 2 and 3; 1 follows 2 but not the other way round

Input:  n = 3, follows = [[0, 1], [0, 1], [1, 0]]
Output: 1
Why:    the repeated pair still describes one mutual pair

Input:  n = 2, follows = []
Output: 0
Why:    edge case, nobody follows anyone

Hints

0 / 3

1. Hint 1reveal
2. Hint 2locked
3. Hint 3locked

def mutual_pairs(n, follows):
    adj = [set() for _ in range(n)]       # adjacency sets: fast edge tests
    for a, b in follows:
        adj[a].add(b)
    count = 0
    for a in range(n):
        for b in adj[a]:
            if a < b and a in adj[b]:     # a < b counts each pair once
                count += 1
    return count

print(mutual_pairs(4, [[0, 1], [1, 0], [1, 2], [2, 3], [3, 2], [0, 2]]))   # -> 2
print(mutual_pairs(3, [[0, 1], [0, 1], [1, 0]]))                           # -> 1
print(mutual_pairs(2, []))                                                 # -> 0