Google Interview Question SDE1s

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    Implementation of Advanced set which have the functionality as "Set" in c++ along with extra functionality-Random number generator.Returns the random number from the set.

    - ANONU on April 14, 2013 in India Report Duplicate | Flag
    Google SDE1

Country: India


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To provide a random() function in addition to the regular "set" interface, we keep a dynamic array (e.g. vector) of the keys in addition to the hashtable which is usually used to store unsorted-set. The index of the item in the array will be stored in the hash-table as the value assoicated with the key.
When a key is added to the set, we append it to the array, and stoe the key in its location in the array in the hash-table. When a key is deleted from the set, we delete it from the array and the hash-table, and then move the last item in the array to where the deleted item was in the array.
To return a random key, we create a random number from 0 to the number of items minus 1, and lookup the corresponding position in the array.

All operations, inclusing the new random() method, will remain O(1).

- gen-y-s on April 20, 2013 | Flag Reply
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Set can be implemented using RB-Tree or AVL Tree. For random number what if we just return the number from any random position from the set.

- Cerberuz on April 14, 2013 | Flag Reply
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And how do you pick a random position in the tree? Presumably the random number generator should have a uniform distrubution over the set.

- Anonymous on April 15, 2013 | Flag
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Randomization can be done like this :
d = random number => tree depth

So we will traverse till depth d, at each node we will generate a random boolean value :
true : move left
false : move right

At last we'll end up on a random node. But the challenge here is uniform distribution of random node selection over all the nodes which looks quiet difficult to me.

- Cerberuz on April 17, 2013 | Flag
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Each node could probably maintain number of children in left and right. Choosing right/left can be a function of these values.
Also proceeding to the next level can also be a function of the depth of the tree.

- bbarodia on April 17, 2013 | Flag
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implement set as AVL tree in array representation.
Find random number based on the index of array.

- nikhil on May 08, 2013 | Flag
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could someone please post a more well defined version of this problem.

- Anonymous on April 15, 2013 | Flag Reply
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We can store the number of node in each sub trees. It will be easy to find the kth element in tree like that. Each time we rand a number k between 1 and N, where N is the total number of the element.

struct Node {
       int val;
       int cnt; // number of nodes in the tree
       Node *left, *rigth;
  }

Node * find_kth(Node *root, int k) {  
     if ( root == NULL || root->cnt < k) {
          return NULL;
      }
      int lc = 0;
      if (root->left != NULL) {
          lc = root->left->cnt; 
     }
     if (lc >= k) {
         return find_kth(root->left, k); 
    } else if (lc == k -1) {
       return root;
   } else{
      return find_kth(root->right, k - lc - 1);
   }
}

- notbad on April 16, 2013 | Flag Reply
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Hi, thanks for the code. I think there might be a minor error. Correct me if I'm wrong please.

if (lc >= k) {
         return find_kth(root->left, k); 
    } else if (lc == k -1) {
       return node;
   } else{
      return find_kth(root->right, k - lc);
   }

I think the final return should be find_kth(root->right, k-lc-1);
Because we also need to deduct their parent node.

- Akira6 on April 19, 2013 | Flag
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@Akria6, yeah, your are right, it should be k-lc-1. Thank you for your correcting. I have re-edited the code.

- notbad on April 19, 2013 | Flag
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if cnt is the number of nodes in the tree, multiple nodes may have the same values, how do you choose from them. Another thing, some k values may not exist in the node's cnt.

- aileen on May 03, 2013 | Flag
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This method is great´╝ü

- 790042744@qq.com on May 10, 2013 | Flag
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else if (lc == k -1) {
       return node;
   }

okay, neat solution, but what exactly is node here , i cant seem to find the declaration

- niku on April 21, 2013 | Flag Reply
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Using a tree to implement the "set" interface is possible like the Java TreeSet class, but it's usually used for ordered sets. For un-ordered set interface it's better to use a hash-table like the java HashSet class, because it provides O(1) performance vs O(log n) performance for TreeSet.

My solution above of using a HashMap and ArrayList together to provide unordered set interface plus a random() method in O(1) time, is thus better than the tree-based implementation you describe as "neat".

- gen-y-s on April 21, 2013 | Flag
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@ niku I'm sorry, it should be

return root;

- notbad on April 23, 2013 | Flag
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of 2 vote

O(1) solution for every set operation and random element of the set.

- Maintain a hash-table for insertion and deletion.
- Along with the hash-table, maintain a vector of pointers to the elements in the hash-table. Lets call it vector(hashtable-element-pointers).
- For insertion, add the element to hash-table, and add a pointer to the end of the vector(hashtable-element-pointers).
- For deletion, get the index of the key of vector(hashtable-element-pointers), replace that pointer with the last element of vector(hashtable-element-pointer) and perform pop_back() on the vector.

Here is the code..

#include <iostream>
#include <unordered_map>
#include <string>
#include <vector>
using namespace std;

// Supports set of strings and random-element in the set.
class mySet {
private:
	unordered_map<string,int> myMap;
	vector<unordered_map<string,int>::iterator> myMapVector;
public:
	mySet() {
		myMap.clear();
		myMapVector.clear();
	}

	bool IsElementPresent(string s) {
		unordered_map<string,int>::iterator it =  myMap.find(s);
		if( it!= myMap.end() )
			return true;			
		return false;
	}

	void Insert( string s ) {
		if( IsElementPresent(s) )
			return;

		myMap[s] = myMapVector.size();
		unordered_map<string,int>::iterator it = myMap.find(s);
		myMapVector.push_back(it);		
	}

	void Delete(string s) {
		if( !IsElementPresent(s) )
			return;

		unordered_map<string,int>::iterator it = myMap.find(s);		
		myMapVector[it->second] = myMapVector[myMapVector.size()-1];
		myMap.erase(it);
		myMapVector.pop_back();
	}

	string GetRandomElement() {
		if(myMapVector.size() == 0 )
			return NULL;

		int randomIndex = rand()%myMapVector.size();
		return myMapVector[randomIndex]->first;
	}

};

int main()
{
	mySet testSet;
	testSet.Insert("zero");
	testSet.Insert("one");
	testSet.Insert("two");
	testSet.Insert("three");
	testSet.Insert("four");
	testSet.Insert("five");
	testSet.Insert("seven");
	testSet.Insert("eight");
	testSet.Insert("nine");

	cout << testSet.IsElementPresent("six") << endl;
	cout << testSet.IsElementPresent("five") << endl;
	cout << testSet.IsElementPresent("four") << endl;	
	testSet.Delete("four");
	cout << testSet.IsElementPresent("four") << endl;
	cout << testSet.GetRandomElement() << endl;
	cout << testSet.GetRandomElement() << endl;
	cout << testSet.GetRandomElement() << endl;
	cout << testSet.GetRandomElement() << endl;
}

- chetan.j9 on May 09, 2013 | Flag Reply
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'Set' could be implemented as a 'hashtable' with chaining as a conflict resolution.

T getRandomElement(){
		
		// choose random bucket [0; buckets length)		
		int randomBucketIndex = rand.nextInt( buckets.length )			
		Bucket bucket = buckets[randomBucketIndex];		
		
		// choose random element from selected bucket, [0; bucket size)
		int randomElemIndex =  rand.nextInt( bucket.size );		
		return bucket.get(randomElemIndex);
	}

- m@}{ on May 14, 2013 | Flag Reply
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How do you implement union operation with this?

- alex on May 14, 2013 | Flag


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