{ "data": { "question": { "questionId": "1705", "questionFrontendId": "1583", "boundTopicId": null, "title": "Count Unhappy Friends", "titleSlug": "count-unhappy-friends", "content": "

You are given a list of preferences for n friends, where n is always even.

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For each person i, preferences[i] contains a list of friends sorted in the order of preference. In other words, a friend earlier in the list is more preferred than a friend later in the list. Friends in each list are denoted by integers from 0 to n-1.

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All the friends are divided into pairs. The pairings are given in a list pairs, where pairs[i] = [xi, yi] denotes xi is paired with yi and yi is paired with xi.

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However, this pairing may cause some of the friends to be unhappy. A friend x is unhappy if x is paired with y and there exists a friend u who is paired with v but:

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Return the number of unhappy friends.

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Example 1:

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\nInput: n = 4, preferences = [[1, 2, 3], [3, 2, 0], [3, 1, 0], [1, 2, 0]], pairs = [[0, 1], [2, 3]]\nOutput: 2\nExplanation:\nFriend 1 is unhappy because:\n- 1 is paired with 0 but prefers 3 over 0, and\n- 3 prefers 1 over 2.\nFriend 3 is unhappy because:\n- 3 is paired with 2 but prefers 1 over 2, and\n- 1 prefers 3 over 0.\nFriends 0 and 2 are happy.\n
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Example 2:

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\nInput: n = 2, preferences = [[1], [0]], pairs = [[1, 0]]\nOutput: 0\nExplanation: Both friends 0 and 1 are happy.\n
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Example 3:

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\nInput: n = 4, preferences = [[1, 3, 2], [2, 3, 0], [1, 3, 0], [0, 2, 1]], pairs = [[1, 3], [0, 2]]\nOutput: 4\n
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Constraints:

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