765. Couples Holding Hands

N couples sit in 2N seats arranged in a row and want to hold hands. We want to know the minimum number of swaps so that every couple is sitting side by side. A swap consists of choosing any two people, then they stand up and switch seats.

The people and seats are represented by an integer from 0 to 2N-1, the couples are numbered in order, the first couple being (0, 1), the second couple being (2, 3), and so on with the last couple being (2N-2, 2N-1).

The couples' initial seating is given by row[i] being the value of the person who is initially sitting in the i-th seat.

Example 1:

Input: row = [0, 2, 1, 3]
Output: 1
Explanation: We only need to swap the second (row[1]) and third (row[2]) person.

Example 2:

Input: row = [3, 2, 0, 1]
Output: 0
Explanation: All couples are already seated side by side.

Note:

1. len(row) is even and in the range of [4, 60].
2. row is guaranteed to be a permutation of 0...len(row)-1.

# @lc code=start
using LeetCode

function min_swaps_couples(row::Vector{Int})
    len, ps, res = length(row), length(row) ÷ 2, 0
    grp = [Set{Int}() for i in 1:ps]
    for i in 1:2:len
        l, r = row[i] ÷ 2 + 1, row[i + 1] ÷ 2 + 1
        (l != r) && (push!(grp[l], r); push!(grp[r], l))
    end
    visited = fill(false, ps)
    q = Queue{Int}()
    for i in 1:ps
        visited[i] && continue
        enqueue!(q, i)
        visited[i] = true
        cnt = 0
        while !isempty(q)
            frt = dequeue!(q)
            cnt += 1
            for neighbor in grp[frt]
                visited[neighbor] && continue
                enqueue!(q, neighbor)
                visited[neighbor] = true
            end
        end
        res += cnt - 1
    end
    res
end
# @lc code=end

min_swaps_couples (generic function with 1 method)

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