310. Minimum Height Trees

package LeetCode_310

import java.util.*
import kotlin.collections.ArrayList

/**
 * 310. Minimum Height Trees
 * https://leetcode.com/problems/minimum-height-trees/description/
 * */
class Solution {
    fun findMinHeightTrees(n_: Int, edges: Array<IntArray>): List<Int> {
        var n = n_
        val result = ArrayList<Int>()
        if (n == 0) {
            return result
        }
        if (n == 1) {
            result.add(0)
            return result
        }

        //init and create graph
        val graph = ArrayList<ArrayList<Int>>()
        for (i in 0 until n) {
            graph.add(ArrayList())
        }
        for (edge in edges) {
            graph.get(edge[0]).add(edge[1])
            graph.get(edge[1]).add(edge[0])
        }

        val queue = LinkedList<Int>()
        for (i in 0 until n) {
            //add leaf node into queue
            if (graph.get(i).size == 1) {
                queue.offer(i)
            }
        }

        //reduce level
        while (n > 2) {
            val size = queue.size
            n -= size
            for (i in 0 until size) {
                val leafNode = queue.poll()//get the leaf node
                for (node in graph[leafNode]) {//find out leaf node‘s connect node
                    graph.get(node).remove(leafNode)//remove the node‘s connection
                    if (graph.get(node).size == 1) {
                        queue.offer(node)
                    }
                }
            }
        }

        while (queue.isNotEmpty()) {
            result.add(queue.poll())
        }

        return result
    }
}