Bulbs | InterviewBit | Solution Explained

I explain the solution to the problem Bulbs in detail, along with visuals and code in Python3.

0. Bulbs: Problem Discussion

Given N bulbs, either on (1) or off (0).

Turning on ith bulb causes all remaining bulbs on the right to flip.

Find the min number of switches to turn all the bulbs on.

0.0. IO Format

• input:
  • an array of binary values (representing bulb states)
• output:
  • the minimum number of switches/flips/cost to turn all the values in the array to 1 (turn the bulbs on).

0.1. Examples

Take for example: [1,0,1].

• the first value is 1, so we continue
• the second value is 0, so
  • we increase the cost by 1
  • flip the remaining
  • the array becomes: [1,1,0]
• the third value is now 0, so
  • we increase the cost by 1
  • flip the remaining (in this case there’s nothing on the right)
  • the array becomes: [1,1,1]
• and thus we get the total cost of 2

1. Bulbs: Observations & Logic

1.0. Intuitions

Using the simple logic mentioned in the question itself, we can see how the simulation will run. It will take O(1) space, since we only store a single variable, and will take O(N*N) time, since we have to:

• iterate over all the bits in the array: O(N)
• for the 0 bits, flipping everything to the right is O(N) – and this is nested inside the above loop

So, how do we proceed to a solution?

1.1. An Interesting Example

Let’s take the case of [0,1,0,1,1,0,1,1]. The underline represents the current element we want to consider and the blue arrows represent the flipping (remember that each costs 1).

There’s something going on in the example, see if you can see it.

Notice the repetition when we align the two arrays!

This should not be all too surprising. Realize that when we flip and convert 1 to 0, we can flip again to convert 0 to 1. This means every two flips, we revert back to the original.

What’s the point of this though?

1.2. Mathematical Formulation

Here’s the simple mathematical representation of what’s happening.

• for any bit b, and at the current cost, cost
• we realize that if the cost is even, we have seen an even number of flips up till now
• this means the bit b has reverted to its original state: b = b
• else, if the cost is odd, we would invert the bit: b = !b
• now, whatever the new b is, we check if its 1 or 0
• if its 1, that means we can continue as before
• else, we have to convert this 0 to a 1, and encounter the cost of 1
• thus, we update the cost: cost = cost + 1 in this case

2. ProblemName: Optimized Implementation

2.0. Code

class Solution:
# @param A : list of integers
# @return an integer
def bulbs(self, A):
cost = 0 # O(1) space

for b in A: # O(N) time
if cost % 2 == 0: b = b
else: b = not b

if b % 2 == 1: continue
else: cost += 1

return cost

2.1. Complexity Analysis

• Time: O(N), to iterate over all the bits in the array
• Space: O(1), to store the cost variable

Tanishq Chaudhary

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