LeetCode/kt/count-the-number-of-complete-components.kt

class Solution {
    private fun makeGraph(
        n: Int,
        edges: Array<IntArray>,
    ): Array<MutableList<Int>> =
        Array<MutableList<Int>>(n) { mutableListOf<Int>() }.let { g ->
            edges.forEach { e ->
                val (u, v) = e[0] to e[1]
                g[u].add(v)
                g[v].add(u)
            }

            g
        }

    private fun dfs(
        graph: Array<MutableList<Int>>,
        visited: BooleanArray,
        u: Int,
    ): Pair<Int, Int> =
        when (visited[u]) {
            true -> 0 to 0
            else -> {
                visited[u] = true
                var (vertices, edges) = 1 to graph[u].size

                graph[u].forEach { v ->
                    dfs(graph, visited, v).let { (newVertices, newEdges) ->
                        vertices += newVertices
                        edges += newEdges
                    }
                }

                vertices to edges
            }
        }

    fun countCompleteComponents(
        n: Int,
        edges: Array<IntArray>,
    ): Int =
        (makeGraph(n, edges) to BooleanArray(n)).let { (g, visited) ->
            visited.indices.asSequence().filter { u -> !visited[u] }.count { u0 ->
                val (vertices, edges) = dfs(g, visited, u0)

                vertices * (vertices - 1) == edges
            }
        }
}