Google Interview Question: Write a routine that does sec... | Glassdoor

Interview Question

Software Engineer Interview Los Angeles, CA

Write a routine that does secret santa in O(N) time.


Interview Answer

4 Answers


is it just a random permutation of an array (done in O(n))?

idispatch on Apr 17, 2013

(random permutation insufficient; must ensure that permutation is not a derangement)

public static ArrayList secretSanta(int N) {
    ArrayList possibleSolution;
    while (true) {
        possibleSolution = new ArrayList();
        ArrayList remainingRecepients = new ArrayList();
        for (int i = 0; i 0) {
            int chosenIndex = r.nextInt(remainingRecepients.size());
            Integer recepient = remainingRecepients.get(chosenIndex);
        boolean valid = true;
        for (int i = 0; i < N; i++) {
            if (possibleSolution.get(i).intValue() == i) {
                valid = false;
        if (valid) break;
    return possibleSolution;

Rahul on May 5, 2013

public ArrayList secretSanta ( int n) {

ArrayList solution = new ArrayList (n);

int remaining = n-1;

Random generator = new Random();

for (int i=0; i<n; i++) {

    int r = generator.nextInt( remaining -i -1) + i+1;
    solution.add ( r );

return solution;

random_consumer on Sep 14, 2013

Best Sol: Shuffle the list, and then split the list into two. Do the pairing within the two groups.
Second Best: Shuffle the list, and then form adjacent pairs from the list. (This solution doesn't give all the permutations)

eleane on Oct 23, 2014

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