How should I practice Count Unhappy Friends?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Bloomberg LP phone screen and onsite interview problems in FastPrep.

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Count Unhappy Friends

Bloomberg LP● MediumNEW GRADPHONE SCREENONSITE INTERVIEW

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Problem statement

There are n people. preferences[x] lists every other person from most to least preferred, and pairs assigns everyone one partner.

Person x is unhappy if there exists u whom x prefers over x's partner y, and u prefers x over u's partner v. Return the number of unhappy people.

Function

unhappyFriends(n: int, preferences: int[][], pairs: int[][]) → int

Examples

Example 1

n = 4preferences = [[1,2,3],[3,2,0],[3,1,0],[1,2,0]]pairs = [[0,1],[2,3]]return = 2

People 1 and 3 satisfy the reciprocal preference condition.

Constraints

2 <= n <= 500 and n is even.
Each preference row is a permutation of all other people.
Pairs form a perfect matching.