There are aliens in n buildings (minimum of 1 in each) and you have to kill all of them minimum number of bombings. Buildings are numbered as 1 – n. Aliens in a bombed building gets injured at the first bombing and die at the second. When a building is bombed for the first time, aliens in that building try to escape to the nearest building (for first building the nearest one is the second one and for nth building it is n-1). Calculate the minimum number of bombs required to kill all the aliens and the order of bombings.
Example:
Input: 3
Output: 4
2 1 3 2
Explanation: Minimum number of bombs required are 4.
First bomb the 2nd building, aliens
will move to 1st or 3rd to save
themselves. Then bomb at 1st building,
if some aliens have moved from 2nd
building to 1st they will be killed and
the 1st building aliens will be injured,
and they will move to the 2nd building
as it is nearest to them. Now, bomb at
the 3rd building to kill aliens who
moved from the 2nd building to 3rd and
injure 3rd building aliens so they move
to 2nd building as it is nearest to them.
Now, bomb at the 2nd building again and
all aliens who moved from 1st or 3rd
building will be killed.
Input: 2
Output: 3
2 1 2
We can make a constructive way to kill all aliens. Since everyone either moves to the left or to the right, we have to make sure that all the even positions are attacked, once at the start to injure aliens and the other time at the end. When we attack aliens at the even positions first time they move to the odd position buildings, so attack them at odd to kill all the previous even positions and injure the odd position aliens. The odd position aliens get injured and will move to the even position, so attack them at the even at the end to kill them.
The number of ways will be n/2 + n/2 + n/2 which is n + n/2.
Below is the implementation of the above approach:
C++
// CPP program to find number of bombings required // to kill all aliens. #include <bits/stdc++.h> using namespace std; // function to print where to shoot void print(int n) { // no. of bombs required cout << n + n / 2 << endl; // bomb all the even positions for (int i = 2; i <= n; i += 2) cout << i << " "; // bomb all the odd positions for (int i = 1; i <= n; i += 2) cout << i << " "; // bomb all the even positions again for (int i = 2; i <= n; i += 2) cout << i << " "; } // driver program int main() { int n = 3; print(n); return 0; } |
Java
// Java program to find number of bombings // required to kill all aliens. class GFG { // function to print where to shoot static void print(int n) { // no. of bombs required System.out.println(n + n / 2); // bomb all the even positions for (int i = 2; i <= n; i += 2) System.out.print( i + " "); // bomb all the odd positions for (int i = 1; i <= n; i += 2) System.out.print(i + " "); // bomb all the even positions again for (int i = 2; i <= n; i += 2) System.out.print( i + " "); } // Driver code public static void main (String[] args) { int n = 3; print(n); } } // This code is contributed by Anant Agarwal. |
Python3
"""Python program to find number of bombings required to kill all aliens""" # function to print where to shoot def bomb_required(n): # no. of bombs required print(n+n // 2) # bomb all the even positions for i in range(2, n + 1, 2): print(i, end = " ") # bomb all the odd positions for i in range(1, n + 1, 2): print(i, end = " ") # bomb all the even positions again for i in range(2, n, 2): print(i, end = " ") # Driver Code bomb_required(3) # This code is contributed by Abhishek Agrawal. |
C#
// C# program to find number of bombings // required to kill all aliens. using System; class GFG { // function to print where to shoot static void print(int n) { // no. of bombs required Console.WriteLine(n + n / 2); // bomb all the even positions for (int i = 2; i <= n; i += 2) Console.Write( i + " "); // bomb all the odd positions for (int i = 1; i <= n; i += 2) Console.Write(i + " "); // bomb all the even positions // again for (int i = 2; i <= n; i += 2) Console.Write( i + " "); } // Driver code public static void Main () { int n = 3; print(n); } } // This code is contributed by vt_m. |
PHP
<?php // PHP program to find number // of bombings required to // kill all aliens. // function to print // where to shoot function p_rint($n) { // no. of bombs required echo floor($n + $n / 2),"\n" ; // bomb all the even positions for ($i = 2; $i <= $n; $i += 2) echo $i ," "; // bomb all the odd positions for ( $i = 1; $i <= $n; $i += 2) echo $i , " "; // bomb all the even positions again for ( $i = 2; $i <= $n; $i += 2) echo $i , " "; } // Driver Code $n = 3; p_rint($n); // This code is contributed by anuj_67. ?> |
Javascript
<script> // javascript program to find number of bombings // required to kill all aliens. // function to print where to shoot function print(n) { // no. of bombs required document.write(n + parseInt(n / 2) + "<br/>"); // bomb all the even positions for (i = 2; i <= n; i += 2) document.write(i + " "); // bomb all the odd positions for (i = 1; i <= n; i += 2) document.write(i + " "); // bomb all the even positions again for (i = 2; i <= n; i += 2) document.write(i + " "); } // Driver code var n = 3; print(n); // This code is contributed by Rajput-Ji </script> |
Output:
4 2 1 3 2
Time complexity: O(n)
Auxiliary Space: O(1)
This article is contributed by Raj. If you like neveropen and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the neveropen main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Ready to dive in? Explore our Free Demo Content and join our DSA course, trusted by over 100,000 neveropen!
