def weighted_union(self, i, j):
        """
        parameters i, j must be roots!
        if size[i] < size[j] then parent[i] = j
        """
        # abs(parent[i]) = size[i]
        # temp_cnt는 음수고 size[i] + size[j]
        temp_cnt = self.parent[i] + self.parent[j]

        # size[i] < size[j]
        if self.parent[i] > self.parent[j]:
            self.parent[i] = j
            self.parent[j] = temp_cnt
        # size[i] > size[j]
        else:
            self.parent[j] = i
            self.parent[i] = temp_cnt