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

boolean array[n]; // The array
int lastTrue = n;
for (int i = n-1; i >= 0; --i) {
  if (array[i]) {
    swap(array[--lastTrue], array[i]);
  }
}

After every iteration all elements after lastTrue are true. No two true elements are swapped because if there was a true element between i and lastTrue it would have been encountered already and moved behind lastTrue. This runs in O(n) time and O(1) memory.