Imagine that we have an array like structure A with n elements all of which are initially 0. ($A[i]=0$)

What is a data structure that supports the following operations:

1) Given an element A[i]=0 set A[i] to 1 in O(1) worst case

2) Given the index i return A[i] in O(1) worst case

3) Given the index i retrun the smallest $j\geq i$ such that $A[j]=0$ or $-1$ if there is no such index in amortized time as small as possible

Obviously an array supports the first 2. It doesnt not support 3). How to modify it?

  • $\begingroup$ Can you only set entries from 0 to 1 or can you also set entries from x to 0? $\endgroup$
    – G. Bach
    Dec 5, 2013 at 2:03
  • $\begingroup$ Only from 0 to 1 $\endgroup$
    – user35202
    Dec 5, 2013 at 4:00
  • $\begingroup$ Yes, this is a nice homework exercise. What have you tried? What's the simplest technique you can think of to avoid repeating work? We expect you to make a serious effort on your own before asking here, and to show us what you've tried. $\endgroup$
    – D.W.
    Dec 6, 2013 at 1:08

1 Answer 1


At every index of the array you should store an extra detail of the immediate index that contains a "0". Updating this index in an amortised complexity of O(1) will give the solution.

Suppose you are updating the extra detail at an index i and the immediate 0 is found at j. Then, the extra detail will have the value j for all the values between i & j.

Now the amortised cost for every operation will be O(1) for updating and retrieving the stored index is O(1).


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