Tuesday 27 August 2019

9 Awesome Puzzles to Churn your Programming Brain

When it comes to sharpening programming skills, puzzles are fun indeed. Puzzles not only help you to learn a new language in a better way but also make you crave for more difficult problems, Bon Appetit! Go ahead and challenge yourself with these awesome hand-picked puzzles for programmers that will put your brains to test and boost your learning process.

Some Popular Puzzles for programmers

Puzzle 1: Tower of Hanoi puzzle

The classic puzzle – the tower of Hanoi is a very famous method to study recursion. The puzzle has three major rules.

  • You have to move one disk at one time
  • You can only move the topmost disk from the tower
  • A large disk can never be upon a smaller disk

Well, you can solve the puzzle in 7 simple steps {as 2n-1 steps where n= number of disks}.
The algorithm for the puzzle is as follows:-

Procedure Hanoi(disk, source, dest, aux)

   IF disk == 0, THEN
      move disk from source to dest         
      Hanoi(disk - 1, source, aux, dest) // Step 1
      move disk from source to dest      // Step 2
      Hanoi(disk - 1, aux, dest, source) // Step 3
END Procedure

 Puzzle 2: The Best Sub-Array Problem

The Best Sub-Array Puzzle is may seem ignorable at first sight but it is tougher than it looks. The puzzle is all about finding the sub-array which sums up to be the largest value.

Note:- make sure that your array contains negative element because if it contains every element as zero or positive, the sum of all slots will be the best sub-array.

The Process:-

Let’s take an example of an array: {1, 1, -5, 1, 1}. You have to always add the first element with the sum of adjacent elements. For example, add 1 with 1and then add 1 with ‘the sum of 1 and -5’, then add 1 with ‘sum of 1, -5, and 1’ and so on.

After reaching the endpoint, discard the first element and perform the process with the other adjacent elements until you reach the endpoint. You will find a subarray to be the best subarray. In this case, there are two slots: [4,5] and [1,2] with the sum as “2”.

Here is the code for the process:-

 best_sum := 0;
  FOR a := 1 TO n
   DO FOR b := 1 TO n
      local_sum := 0;
      FOR i := a TO b
        DO local_sum := local_sum + Slots[i];
        best_sum :=
      GREATEST (local_sum, best_sum);

Puzzle 3: Three Warehouse Puzzles

The puzzle of three warehouse tests your reasoning where you have to shift elements from a warehouse in the number of three where you have two distribute two elements to other warehouses and keep one for yourself (you have to discard that element from the puzzle).

You have to move three clips from a warehouse in such a way that there is at least 0 clip in the warehouse. You have to distribute clips to the other two warehouses by eliminating one clip every time. Continue the process until you reach a point where you do not have three clips to move from a warehouse.

Here I have taken an example of three warehouses A, B and C with the count of 3, 1, 4 clips. I start the process with warehouse A where I have moved 3 clips, distributed it to others (B and C) and eliminated one. The process gives the count of (0, 2, 5). The further processes give results like (1, 3, 2), (2, 0, 3), (3,1, 0), and (0, 2, 1). All warehouses have less than 3 clips. Hence, the process stops.
You can prepare your own warehouse and test your programming skills.

Puzzle 4: The Rickety Bridge Problem

Four travelers have to cross a shaky bridge at night. The bridge can only bear the weight of two persons at a given time and a flashlight is needed to cross the bridge. There is only one flashlight available among the group.

Now, no traveler walks with the same speed: it takes 1 minute for the first traveler to cross the bridge, whereas it takes 2, 5 and 10 minutes for the second, third and fourth traveler to perform the same task. Moreover, while crossing the bridge in a set of two, the faster traveler has to match the pace of the slower ones. Hence, how much time (least amount) will it take for all travelers to cross the bridge?

It will take 17 minutes for travelers to perform the task. How? Let’s find out.

  • The first and the second traveler can cross in 2 minutes – 2 minutes
  • The second traveler comes back in 2 minutes- 4 minutes
  • Now, the third and fourth cross in 10 minutes –14 minutes
  • The first comes back –15 minutes
  • Then, the first and second cross together within 2 minutes, making it a total of 17 minutes.

Puzzle 5: The Lake, the Goblin and You

You are in the center of a circular lake and a goblin is waiting on the shore to kill you. The goblin never eats, sleeps and cannot swim but has got great eyesight. He is logical and will do anything which is possible to get you.

Now, you can outrun the goblin on land but he can run four times faster than your boat. How will you escape him?

It’s simple.

r= radius of circular lake
e= very small distance

Paddle r/4 – e away from the center of the lake. Maintain the distance and now, paddle around center of the lake as long as you reach the side opposite to goblin. Hence, the goblin would have to run faster than his maximum speed which is not possible. Now, paddle in a straight path against the location of goblin towards the nearest shore. Paddle up to the distance of   ¾ * r + e whereas goblin has to run more than four times faster which is pi * 4.

Puzzle 6: The Perfect Murder

A person shoots his wife. Then she holds her underwater for 5 minutes. Finally, he hangs her. But after 10 minutes they both go out together and enjoy a wonderful dinner together. How can this be?
The person is a photographer. He takes his wife’s picture (shot her), held her picture in water for 5 minutes (held her in the water) and hanged it to dry (hanged her). After 10 minutes, he went out with her for dinner.

Puzzle 7: The Birthday Cake Problem

A circular birthday cake has to be divided into 8 equal pieces within 3 cuts. How can you do it?
Cut the cake in the horizontal and vertical direction. Now as your cake is cut into four equal pieces, arrange these pieces in a stack. Afterward, cut the stack in half. Thus, you will have 8 equal pieces.

Puzzle 8: Where Did The Logic Go?

Here’s a code in PHP which asks input from the user and checks whether it is in range or not.

if($max<20 and $min>15)
echo 'in range';
 echo 'out range';

Thus, if we pass values 15 and 10, $max=15 and $min=10, which means $max<20 is true and $min>15 is false. It will show true and false.



then the logical expression:
$inrange = $a and $b;

Hence, it can be written as:-

echo 'in range';
echo 'out range';

Now, when you compile the program with the same inputs, it shows $inrange=true whereas it should have shown the expression as false.. How did it happen?


Here, $inrange=$a and $b;
is considered as
($inrange=$a) and $b;

Hence, as assignment is evaluated as a high priority operation, it is performed earlier.
Thus, contrary to the expectations, the in-range value is shown as true because the in-range value is evaluated as $a and not “$a and $b”.

Hence, to solve this problem, you have to replace “$inrange=$a and $b” with “$inrange=($a and $b)”.

Puzzle 9: The False String

Here is a code in PHP which says:-

if ($result=strpos($string,$target))
echo("found ".$result);
echo("not found ".$result);

The ideal result should have found the target and considered the value as true. But, the result is shown as “not found”. Why?

Let’s find it out:-

Do you remember that all values convert to true except for:-

  • SimpleXML objects created from empty tags
  • the special type NULL (including unset variables)
  • an object with zero member variables (PHP 4 only)
  • an array with zero elements
  • the empty string, and the string “0”
  • the float 0.0 (zero)
  • the integer 0 (zero)
  • the boolean FALSE itself

Hence, in this case, as our target is ‘a’ in the above code,  the value will get converted in False and strpos will be zero and the message will be ‘not found’.

The solution to this problem is:-

if (($result=strpos($string,$target))!==false)
echo("found ".$result);
echo("not found ".$result);
In this code, “!==” checks the type of the value and makes sure that it is not false, thus preventing it from type juggling and converting into false.

Elongate this list by sharing your favorite puzzles or share your experience while solving these puzzles here in the comments. Always remain adventurous.

Happy Coding!

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