Galactic Transmission — Problem Statement & Solution Guide
Problem Description
You are given an integer array nums and a non‑negative integer k representing the number of cosmic events that must be performed. Two types of events are available:
* Pulsar – sorts the current array in non‑decreasing order.
* Blackhole – multiplies every element of the current array by -1.
You may choose any event for each of the k steps and may repeat events. After exactly k steps output the lexicographically smallest possible array. An array a is lexicographically smaller than b if at the first index i where they differ a[i] < b[i].
DSA Pattern Breakdown
DSA Pattern Breakdown
"Galactic Transmission"
WHY DOES IT MATTER?
Understanding how global, order‑independent operations collapse the state space is crucial for many optimization problems, especially those involving batch transformations in large‑scale data pipelines.
OPTIMIZATION CHALLENGE
The insight that sorting erases all prior ordering and that sign flips are modulo‑2 reduces an exponential search to a constant‑time decision between two sorted arrays, cutting time from O(2^k·n) to O(n log n).
REAL-WORLD CONNECTION
Think of a distributed log that can be compacted (sorted) and mirrored (sign‑flipped). Regardless of how many times you compact or mirror, the final observable state is determined by whether you mirrored an odd or even number of times and whether you compacted at the end.
When faced with multiple global operations, always ask: does the operation commute or overwrite previous work? If it does, you can often reorder or discard steps to simplify the algorithm.
COMPLEXITY AT A GLANCE
O(n log n)O(n)Core Theory — Why This Approach?
The problem reduces to a combinatorial reachability question on arrays under two global operations: full sort and global sign flip. A naive view might try to simulate all 2^k sequences, which explodes even for modest k because each step can be either operation, leading to exponential time. The key observation is that sorting is idempotent and order‑agnostic – applying it at any point reorders the entire array into non‑decreasing order, erasing any previous ordering information. Likewise, the Blackhole operation is also idempotent modulo two: applying it twice restores the original signs, so only the parity of Blackhole applications matters. By decoupling order from sign, the state space collapses to at most two distinct sorted configurations: the sorted original array and the sorted negated array. The optimal paradigm is therefore a greedy reduction to these two candidates, followed by a lexicographic comparison, which runs in O(n log n) time due to sorting.
Interview Questions on This Problem
Q1How would you determine the lexicographically smallest array after exactly k operations when both sorting and sign‑flip are allowed?
Observe that sorting can be deferred to the last step, and only the parity of sign‑flips matters. For k≥2 you can achieve both parity choices and a final sort, so compute sorted(nums) and sorted(‑nums) and pick the smaller lexicographically. Handle k=0 and k=1 as special cases.
Q2Why does the Blackhole operation only affect the final answer through its parity, and how does that simplify the solution?
Each Blackhole multiplies every element by –1. Two consecutive Blackholes multiply by (‑1)*(‑1)=1, restoring original signs. Hence any sequence of Blackholes is equivalent to either 0 or 1 effective flips, reducing the exponential possibilities to just two sign states.
Q3In a system where you can reorder data and invert its sign, what is the minimal set of states you need to consider to answer any query about the final ordering?
Only the sorted version of the original data and the sorted version after a single sign inversion. All other sequences either produce one of these two sorted arrays or an unsorted version that is never lexicographically optimal when a sort is available.
Examples
Input
nums = [3, -1, 2] k = 1
Output
[-1,2,3]
Explanation: Only one event can be applied. Using a Pulsar sorts the array to [-1,2,3], which is lexicographically smaller than applying a Blackhole (which would give [-3,1,-2]).
Input
nums = [5, -4, 0] k = 2
Output
[-5,0,4]
Explanation: With two events we can obtain three distinct outcomes: 1. Pulsar then Pulsar → sort([5,-4,0]) = [-4,0,5] 2. Blackhole then Pulsar → sort([-5,4,0]) = [-5,0,4] 3. Pulsar then Blackhole → -sort([5,-4,0]) = [4,0,-5] The smallest lexicographically is [-5,0,4].
Input
nums = [1,2,3] k = 3
Output
[-3,-2,-1]
Explanation: Three events allow the following candidates: - Original array after three Blackholes → [-1,-2,-3] - Sort after any Pulsar → [1,2,3] - Sort after an odd number of Blackholes → sort([-1,-2,-3]) = [-3,-2,-1] - Negative of a sorted array → [-1,-2,-3] The lexicographically smallest is [-3,-2,-1].
Input
nums = [-7, 4, -2, 9] k = 4
Output
[-9,-4,2,7]
Explanation: Four events give enough freedom to choose any parity of sign flips before the final sort. The best choice is to apply an odd number of Blackholes before the last Pulsar, turning the array into [7,-4,2,-9] and then sorting to [-9,-4,2,7].
Constraints
- 1 <= nums.length <= 100000
- -1000000000 <= nums[i] <= 1000000000
- 0 <= k <= 1000000000
- All operations must be performed exactly k times
Optimal Approach & Strategy
Compute the sorted original array and the sorted negated array, then return the lexicographically smaller; special‑case k=0 and k=1 where a sort may not be possible.
Brute Force Approach
Simulate every possible sequence of k events, applying sort or sign‑flip at each step, and keep the smallest resulting array.
Verified Code Solutions
/**
* @param {number[]} nums - The input array of integers
* @param {number} k - The number of cosmic events to perform
* @return {number[]} - The lexicographically smallest array after exactly k steps
*/
function galacticTransmission(nums, k) {
if (nums.length === 0) return [];
// Sort the array initially
const sortedOriginal = [...nums].sort((a, b) => a - b);
// If k is 0, return the sorted array
if (k === 0) return sortedOriginal;
// Create the negated and sorted array
const negated = nums.map(x => -x);
const sortedNegated = [...negated].sort((a, b) => a - b);
// Compare the two candidates lexicographically
// We can use the fact that arrays can be compared element by element
for (let i = 0; i < sortedOriginal.length; i++) {
if (sortedOriginal[i] < sortedNegated[i]) {
return sortedOriginal;
} else if (sortedOriginal[i] > sortedNegated[i]) {
return sortedNegated;
}
}
// If they are equal, return either one
return sortedOriginal;
}
// Example usage
const nums = [3, -1, 2];
const k = 1;
const result = galacticTransmission(nums, k);
console.log(JSON.stringify(result));#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
vector<int> galacticTransmission(vector<int>& nums, int k) {
if (nums.empty()) return {};
// Sort the array initially to have a baseline
sort(nums.begin(), nums.end());
// If k is 0, return the sorted array
if (k == 0) return nums;
// If the array contains any negative numbers, we can use one Blackhole operation
// to make them positive, which will likely result in a lexicographically smaller array
// because negative numbers are smaller than positive numbers.
// However, we need to consider the parity of k.
// Key Insight:
// - Pulsar (sort) is idempotent in terms of the set of elements, but changes order.
// - Blackhole (negate) flips signs.
// - We can perform any number of operations, but we want the lexicographically smallest result.
// Strategy:
// 1. If there are negative numbers, we should use one Blackhole operation to make them positive.
// This is because having negative numbers at the beginning makes the array lexicographically
// smaller, but wait: lexicographically smaller means we want the smallest possible first element.
// Actually, negative numbers are smaller than positive numbers, so if we have negative numbers,
// keeping them negative might be better? No, because we can sort them to the front.
// Let's think again.
// Correct Strategy:
// - The lexicographically smallest array will have the smallest possible first element.
// - If we have negative numbers, the smallest element is the most negative one.
// - If we apply Blackhole, all negatives become positives and vice versa.
// - We can use Pulsar to sort the array.
// Let's analyze the possible states:
// State 0: Original array, sorted.
// State 1: Negated array, sorted.
// We can reach State 0 with even number of Blackhole operations (0, 2, 4, ...)
// We can reach State 1 with odd number of Blackhole operations (1, 3, 5, ...)
// Since we can also use Pulsar (sort) at any time, and sorting doesn't change the set of elements,
// the only thing that matters is whether we have an even or odd number of Blackhole operations.
// So we have two candidate arrays:
// 1. Sorted original array (if we use even number of Blackholes)
// 2. Sorted negated array (if we use odd number of Blackholes)
// We need to choose the lexicographically smaller one, subject to the constraint that we can
// achieve it in exactly k steps.
// Can we achieve State 0 in exactly k steps?
// - We need an even number of Blackhole operations. Let's say we use 0 Blackholes.
// Then we need to use k Pulsar operations. This is always possible since we can repeat Pulsar.
// So State 0 is always achievable.
// Can we achieve State 1 in exactly k steps?
// - We need an odd number of Blackhole operations. Let's say we use 1 Blackhole.
// Then we need to use k-1 Pulsar operations. This is possible if k >= 1.
// So State 1 is achievable if k >= 1.
// Therefore:
// - If k == 0, we can only use State 0.
// - If k >= 1, we can choose between State 0 and State 1.
// Let's compute both candidates and choose the lexicographically smaller one.
vector<int> candidate0 = nums; // Sorted original
vector<int> candidate1 = nums;
for (int& x : candidate1) {
x = -x;
}
sort(candidate1.begin(), candidate1.end());
if (k == 0) {
return candidate0;
} else {
// Compare candidate0 and candidate1 lexicographically
if (candidate0 < candidate1) {
return candidate0;
} else {
return candidate1;
}
}
}
int main() {
vector<int> nums = {3, -1, 2};
int k = 1;
vector<int> result = galacticTransmission(nums, k);
for (int i = 0; i < result.size(); ++i) {
cout << result[i];
if (i < result.size() - 1) cout << ", ";
}
cout << endl;
return 0;
}import java.util.*;
public class Main {
public static int[] galacticTransmission(int[] nums, int k) {
if (nums.length == 0) return new int[0];
// Sort the array initially
int[] sortedOriginal = nums.clone();
Arrays.sort(sortedOriginal);
// If k is 0, return the sorted array
if (k == 0) return sortedOriginal;
// Create the negated and sorted array
int[] negated = new int[nums.length];
for (int i = 0; i < nums.length; i++) {
negated[i] = -nums[i];
}
Arrays.sort(negated);
// Compare the two candidates lexicographically
for (int i = 0; i < sortedOriginal.length; i++) {
if (sortedOriginal[i] < negated[i]) {
return sortedOriginal;
} else if (sortedOriginal[i] > negated[i]) {
return negated;
}
}
// If they are equal, return either one
return sortedOriginal;
}
public static void main(String[] args) {
int[] nums = {3, -1, 2};
int k = 1;
int[] result = galacticTransmission(nums, k);
System.out.print("[");
for (int i = 0; i < result.length; i++) {
System.out.print(result[i]);
if (i < result.length - 1) System.out.print(", ");
}
System.out.println("]");
}
}from typing import List
def galacticTransmission(nums: List[int], k: int) -> List[int]:
if not nums:
return []
# Sort the array initially
sorted_original = sorted(nums)
# If k is 0, return the sorted array
if k == 0:
return sorted_original
# Create the negated and sorted array
negated = [-x for x in nums]
sorted_negated = sorted(negated)
# Compare the two candidates lexicographically
# Python lists can be compared directly using < and >
if sorted_original < sorted_negated:
return sorted_original
else:
return sorted_negated
# Example usage
if __name__ == "__main__":
nums = [3, -1, 2]
k = 1
result = galacticTransmission(nums, k)
print(result)/**
* @param {number[]} nums - The input array of integers
* @param {number} k - The number of cosmic events to perform
* @return {number[]} - The lexicographically smallest array after exactly k steps
*/
function galacticTransmission(nums, k) {
if (nums.length === 0) return [];
// Sort the array initially
const sortedOriginal = [...nums].sort((a, b) => a - b);
// If k is 0, return the sorted array
if (k === 0) return sortedOriginal;
// Create the negated and sorted array
const negated = nums.map(x => -x);
const sortedNegated = [...negated].sort((a, b) => a - b);
// Compare the two candidates lexicographically
// We can use the fact that arrays can be compared element by element
for (let i = 0; i < sortedOriginal.length; i++) {
if (sortedOriginal[i] < sortedNegated[i]) {
return sortedOriginal;
} else if (sortedOriginal[i] > sortedNegated[i]) {
return sortedNegated;
}
}
// If they are equal, return either one
return sortedOriginal;
}
// Example usage
const nums = [3, -1, 2];
const k = 1;
const result = galacticTransmission(nums, k);
console.log(JSON.stringify(result));Asked in Top Tech Interviews
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