Boolean array reordering in O(1) space and O(n) time

Question

The problem is taken from Elements of Programming Interviews:

Given an array A of n objects with Boolean-valued keys, reorder the array so that objects that have the

Answer 1

Let the array have 0 based index and let it have n elements. Then you can do the following ( pseudo code below )

     // A[] is your array
     i = 0
     k = 0
     for i from 0 to n-1
          if A[i] is true
             swap A[i] and A[k]
             increment k

Time complexity is O(n) and extra space is only used for two variables i and j so memory is O(1). This way ordering is preserved amongst false and true values. ( This method puts true ones first you can change it according to your requirement ).