Question

Do problem

4-20: Give an efficient algorithm to rearrange an array of n keys so that all the negative keys precede all the nonnegative keys. Your algorithm must be in-place, meaning you cannot allocate another array to temporarily hold the items. How fast is your algorithm?

Answer 1

This problem can be solved in O(N) time.

We need two pointers, I and J starting from left and right. If Element from left is positive and Element from Right is Negative, then, we need to swap those two elements

FUN REARRANGE(A[])
   N = LEN(A)
   I = 0
   J = N-1
   WHILE I<J
       WHILE I<N && A[I]<0
           I++
       WHILE J>=0 && A[J]>0
           J--
       IF I<N && J>=0
           T = A[I]
           A[I] = A[J]
           A[J] = T