Cut property를 이용.

import Foundation
var dict = [Int: [[Int]]]()
func solution(_ n:Int, _ costs:[[Int]]) -> Int {
    for cost in costs {
        if let _ = dict[cost[0]] {
            dict[cost[0]]!.append( [cost[1], cost[2]])
        } else {
            dict[cost[0]] = [[cost[1], cost[2]]]
        }
        if let _ = dict[cost[1]] {
            dict[cost[1]]!.append([cost[0], cost[2]])
        } else {
            dict[cost[1]] = [[cost[0], cost[2]]]
        }
    }
    
    return prim(len: n)
}

func prim(len: Int) -> Int {
    var f = Array(repeating: false, count: len)
    var pq = [Int]()
    var cost = Array(repeating: Int.max, count: len)
    var e = Array(repeating: -1, count: len)
    //
    var t = [[Int]]()
    for v in 0..<len {
        pq.append(v)
    }
    while !pq.isEmpty {
        pq.sort { cost[$0] > cost[$1] }
        let v = pq.removeLast()
        f[v] = true
        if e[v] != -1 {
            print(e[v], v)
            t.append([e[v], v])
        }
        
        for next in dict[v]! {
            if f[next[0]] == false && next[1] < cost[next[0]] {
                cost[next[0]] = next[1]
                e[next[0]] = v
            } 
        }
    }
    var ans = 0
    for edge in t {
        for next in dict[edge[0]]! {
            if next[0] == edge[1] {
                ans += next[1]
                break
            }
        }
    }
    //print(t)
    return ans 
}