Problem p02558 (python)

Fixed submission:
import typing


class DSU:
    '''
    Implement (union by size) + (path compression)
    Reference:
    Zvi Galil and Giuseppe F. Italiano,
    Data structures and algorithms for disjoint set union problems
    '''

    def __init__(self, n: int = 0):
        self._n = n
        self.parent_or_size = [-1] * n

    def merge(self, a: int, b: int) -> int:
        assert 0 <= a < self._n
        assert 0 <= b < self._n

        x = self.leader(a)
        y = self.leader(b)

        if x == y:
            return x

        if -self.parent_or_size[x] < -self.parent_or_size[y]:
            x, y = y, x

        self.parent_or_size[x] += self.parent_or_size[y]
        self.parent_or_size[y] = x

        return x

    def same(self, a: int, b: int) -> bool:
        assert 0 <= a < self._n
        assert 0 <= b < self._n

        return self.leader(a) == self.leader(b)

    def leader(self, a: int) -> int:
        assert 0 <= a < self._n

        if self.parent_or_size[a] < 0:
            return a

        self.parent_or_size[a] = self.leader(self.parent_or_size[a])
        return self.parent_or_size[a]

    def size(self, a: int) -> int:
        assert 0 <= a < self._n

        return -self.parent_or_size[self.leader(a)]

    def groups(self) -> typing.List[typing.List[int]]:
        leader_buf = [self.leader(i) for i in range(self._n)]

        result = [[] for _ in range(self._n)]
        for i in range(self._n):
            result[leader_buf[i]].append(i)

        return list(filter(lambda r: r, result))


# https://atcoder.jp/contests/practice2/tasks/practice2_a
def main() -> None:
    import sys

    n, q = map(int, sys.stdin.readline().split())
    dsu = DSU(n)

    for _ in range(q):
        t, u, v = map(int, sys.stdin.readline().split())
        if t == 0:
            dsu.merge(u, v)
        if t == 1:
            if dsu.same(u, v):
                print(1)
            else:
                print(0)


if __name__ == '__main__':
    main()

Buggy submission (verdict: Runtime Error):
// package jp.atcoder.library.kotlin.dsu

import java.util.*

/**
 * Disjoint set union.
 */
class DSU(private val n: Int) {
    private val parentOrSize = IntArray(n)

    /**
     * Merge nodes.
     */
    fun merge(a: Int, b: Int): Int {
        if (a !in 0 until n) throw IndexOutOfBoundsException("a=$a")
        if (b !in 0 until n) throw IndexOutOfBoundsException("b=$b")
        var x = leader(a)
        var y = leader(b)
        if (x == y) return x
        if (-parentOrSize[x] < -parentOrSize[y]) {
            val tmp = x
            x = y
            y = tmp
        }
        parentOrSize[x] += parentOrSize[y]
        parentOrSize[y] = x
        return x
    }

    /**
     * True if two nodes are connected.
     */
    fun same(a: Int, b: Int): Boolean {
        if (a !in 0 until n) throw IndexOutOfBoundsException("a=$a")
        if (b !in 0 until n) throw IndexOutOfBoundsException("b=$b")
        return leader(a) == leader(b)
    }

    /**
     * Get its leader node.
     */
    fun leader(a: Int): Int {
        return if (parentOrSize[a] < 0) {
            a
        } else {
            parentOrSize[a] = leader(parentOrSize[a])
            parentOrSize[a]
        }
    }

    /**
     * A group's size.
     */
    fun size(a: Int): Int {
        if (a !in 0 until n) throw IndexOutOfBoundsException("" + a)
        return -parentOrSize[leader(a)]
    }

    /**
     * Group by leader.
     */
    fun groups(): ArrayList<ArrayList<Int>> {
        val leaderBuf = IntArray(n)
        val groupSize = IntArray(n)
        for (i in 0 until n) {
            leaderBuf[i] = leader(i)
            groupSize[leaderBuf[i]]++
        }
        val result = ArrayList<ArrayList<Int>>(n)
        for (i in 0 until n) {
            result.add(ArrayList(groupSize[i]))
        }
        for (i in 0 until n) {
            result[leaderBuf[i]].add(i)
        }
        result.removeIf { obj: ArrayList<Int> -> obj.isEmpty() }
        return result
    }

    init {
        Arrays.fill(parentOrSize, -1)
    }
}

private fun readLn() = readLine()!! // string line
private fun readInt() = readLn().toInt() // single int
private fun readStrings() = readLn().split(" ") // list of strings
private fun readInts() = readStrings().map { it.toInt() } // list of ints

fun main(args: Array<String>) {
    val (n, q) = readInts()
    val uf = DSU(n)

    repeat(q) {
        val (t, u, v) = readInts()
        if (t == 0)
            uf.merge(u, v)
        else
            println(if (uf.same(u, v)) 1 else 0)
    }
}