Challenge – First Common Ancestor

Question: How would you find the first common ancestor of two nodes in a binary search tree? First as in the lowest in the tree. Another way to ask is to find the lowest common ancestor of two nodes.

Challenge: Do you know the answer to this question? Post in the comments. Answers will be posted March 6th.

Meanwhile, check out the challenges from previous weeks here.


TreeNode findFirstCommonAncestor(TreeNode root, TreeNode p,
	TreeNode q) {

    if (root == null) {
        return null;

    if (root == p || root == q) {
	return root;

    TreeNode left = findFirstCommonAncestor(root.left, p, q);
    TreeNode right = findFirstCommonAncestor(root.right, p, q);

    if ((left == p && right == q) ||
        (left == q && right == q)) {
	return root;

    return (left != null) ? left : right;


TreeNode findFirstCommonAncestor(TreeNode root, int p, int q) {

    if (root == null) {
        return null;

    if (root.value == p || root.value == q) {
	return root;

    if (root.value > p && root.value > q ) {
        return findFirstCommonAncestor(root.left, p, q);
    else if (root.value < p && root.value < q ) {
        return findFirstCommonAncestor(root.right, p, q);
    else {
        return root;

Thanks Dave and Sunil for pointing out the alternate solution.

Challenge Series

Hey everyone,

We got great response from the interview challenge posted last week. We’ve decided to make a weekly challenge series (hopefully we have enough questions to keep up :)).

Do you have any interesting questions you would like to share? Or any questions that you’d like answered? Let us know and we can help draw the audience to answer your questions!



Balanced Tree

Question: How would you check if a binary tree is balanced?
Answer: A tree is considered balanced when the difference between the min depth and max depth does not exceed 1.

Recursive algorithms always work well on trees, so here’s some code.

int min_depth( Node * root ) {
    if( !root ) {
        return 0;
    return 1 + min( min_depth( root->left ), 
                    min_depth( root->right ));

int max_depth( Node * root ) {
    if( !root ) {
        return 0;
    return 1 + max( max_depth( root->left ), 
                            max_depth( root->right ));

bool is_balanced( Node * root ) {
    return ( max_depth( root ) - min_depth( root ) ) <= 1

Simple enough, right? Spread the word on Twitter and Digg!



Queue Using Stacks

Question: How would you use stacks to implement a queue?

Answer: So how could we go about doing this? What if we used two stacks and used one as the incoming stack and the other as the outgoing stack? A queue is a FIFO structure, so when we enqueue something, we need to make sure that it does not get popped off before something that was already there. Similarly, when we dequeue something, we have to make sure that elements that were inserted earlier get ejected first.

What’s better than some code!

Stack in;
Stack out;
void enqueue( int value ) {
    while( !out.isEmpty() ) {
        in.push( out.pop() );
    in.push( value );

int dequeue() {
    while( !in.isEmpty() ) {
        out.push( in.pop() );
    return out.pop();

Isn’t that cool? Help share this on Twitter!


Bit Swaps

Question: How would you find the number of bit swaps required to convert integer A to integer B?

Answer: Gut instinct might suggest that you go through every bit of A and every bit of B while simultaneously updating a count of the bits that are different. But how about a cooler way to solve this?

A key to solving a lot of bit manipulation questions is the use of the XOR functionality.

int bit_swaps_required( int a, int b ) {
    unsigned int count = 0;
    for( int c = a ^ b; c != 0; c = c >> 1 ) {
        count += c & 1;
    return count;

Simple right?

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Challenge – Equal Probability between 1 and 7

Question: Write a method to generate a random number between 1 and 7, given a method that generates a random number between 1 and 5. The distribution between each of the numbers must be uniform.

Challenge: Do you know the answer to this question? Post in the comments. Let’s see who is up for the challenge. Answers will be posted February 26th.

Let’s think of this like a decision tree. Each rand5() will be a decision. After 2 tries, we have 25 possible solutions. We try to get maximum bunches of 7 as we can (1 – 21, 3 sets of 7). If we get any of the other values, we just try again. Since the probability of getting each of 21 values are the same every time, trying again won’t affect their probabilities.

Equal Probability 1 to 7

int rand7() {
    while (1) {
        int num = 5*(rand5()-1) + rand5();
        if (num < 22) return ((num % 7) + 1);

That was fun, right? Anyone up for another challenge? Watch out for it next tuesday (March 1st).


Card Pairology?

Question: John and Matt decide to play a game using a deck of cards. The game involves flipping two cards at a time. If both cards are black, they go into John’s pile and if both cards are red, they go into Matt’s pile. If one card is red and one card is black, they go into a discard pile. The game is played until all 52 cards have been flipped. The person with the most cards in their pile wins. If there is a tie, John wins. If Matt has more cards than John, then John pays Matt a dollar. What is the chance of Matt getting a dollar?

Answer: Any guesses? The chance of Matt getting any money is zero. Do you know why?

Say the deck is arranged in a way such that there are 13 black pairs. In this situation, John gets 13 black pairs. And since there are no other black cards left, Matt gets 13 red pairs too. So there’s a tie and John wins.

Suppose there are 12 black pairs in the deck. In this case, there would be 2 black cards left in the deck that would pair up with 2 other red cards in the deck. These 2 black cards would never be together since we have already claimed that there are only 12 black pairs in the deck. As a result, 2 pairs of cards end up in the discard pile. The remaining cards would all be red and Matt would get 12 red pairs too. So once again, there’s a tie and John wins.

We could continue this process through induction. Assuming there are 11 black pairs, there would be 4 other black cards that pair up with 4 other red cards and go into the discard pile. Once again, both John and Matt end up with 11 pairs of cards each and John wins.

Here’s a table to explain each case.

John Discarded Matt
13 pairs 0 pairs 13 pairs
12 pairs 2 pairs 12 pairs
11 pairs 4 pairs 11 pairs
10 pairs 6 pairs 10 pairs
9 pairs 8 pairs 9 pairs
8 pairs 10 pairs 8 pairs
7 pairs 12 pairs 7 pairs
6 pairs 14 pairs 6 pairs
5 pairs 16 pairs 5 pairs
4 pairs 18 pairs 4 pairs
3 pairs 20 pairs 3 pairs
2 pairs 22 pairs 2 pairs
1 pair 24 pairs 1 pair
0 pairs 26 pairs 0 pairs

So we see that in any case, there is a tie between John and Matt. So Matt never ends up with a dollar.

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Boxes of Money

Question: You are given b boxes and n dollar bills. The money has to be sealed in the b boxes in a way such that without thereafter opening a box, you can give someone a requested whole amount of dollars from 0 to n. How should b be related to n for this to happen?

Answer: Stumped? Let’s think of an example to approach this problem.

Say we have $100. A good approach to distributing $100 would be the binary number system. So you’d have $1, $2, $4, $8, $16, $32 in the first six boxes. We can’t fill the next box with $64 dollars because we are only left with $37 dollars (from a total of $100). So we’d have to put $37 in the seventh box. To supply any requested amount, we’d have to use a combination of these boxes.

To find out the restrictions on the values of b and n, we have to think of different scenarios. For instance, with a million dollars and just one box, we would never be able to dispense any requested amount less than a million. However, if we are ever in a situation with more boxes than dollars, there is a never a problem.

Using this approach, we can create a table showing the best relationship between b and n

b = 1     n = up to $1
b = 2     n = up to $2 + $1 = $3
b = 3     n = up to $4 + $2 + $1 = $7
b = 4     n = up to $8 + $4 + $2 + $1 = $15

See a pattern yet? So the best way we would be able to dispense any requested amount is to have n <= 2^b – 1.

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Chasing Dogs

Question: There are four dogs each at the corner of a unit square. Each of the dogs starts chasing the dog in the clockwise direction. They all run at the same speed and continuously change their direction accordingly so that they are always heading straight towards the other dog. How long does it take for the dogs to catch each other and where?

Answer: Let the dogs be A, B, C and D where A is chasing B, B is chasing C, C is chasing D and D is chasing A.

All the dogs will eventually meet in the center of the square. Since all the dogs move in symmetry, the only logical answer to the location of their meeting is the center of the square.

At any point in time, dog A is perpendicular to dog B and B perpendicular to C and so on. Dog A moves towards dog B but dog B does not move towards or away from dog A since it is moving perpendicular to dog A. Therefore, the distance that dog A needs to cover to reach dog B is the same as the original distance between them, one unit.

The speed of each of the dog towards the dog it is chasing is given by (1 + cos(t)) where t is the angle on each corner of the square.

Speed of dog = 1 + cos(90) = 1 + 0 = 1
Time needed = Distance/Speed = 1 / 1 = 1 unit.

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Have a better solution? Let us know through the comments section!


Red and Blue Marbles

Question: You have 50 red marbles, 50 blue marbles and 2 jars. One of the jars is chosen at random and then one marble will be chosen from that jar at random. How would you maximize the chance of drawing a red marble? What is the probability of doing so? All 100 marbles should be placed in the jars.

Answer: Seems tricky at first right? Given that the number of red and blue marbles are the same, you would tend to think that the odds are 50-50. You would try different combinations, such as 25 of each colored marble in a jar or putting all red marbles in one jar and all the blue in the other. You would still end up with a chance of 50%.

So lets think of a better way to distribute the marbles. What if you put a single red marble in one jar and the rest of the marbles in the other jar? This way, you are guaranteed at least a 50% chance of getting a red marble (since one marble picked at random, doesn’t leave any room for choice).  Now that you have 49 red marbles left in the other jar, you have a nearly even chance of picking a red marble (49 out of 99).

So let’s calculate the total probability.

P( red marble ) = P( Jar 1 ) * P( red marble in Jar 1 ) + P( Jar 2 ) * P( red marble in Jar 2 )
P( red marble ) = 0.5 * 1 + 0.5 * 49/99
P( red marble ) = 0.7474

Thus, we end up with ~75% chance of picking a red marble.

Have a better solution? Let us know through the comments section!