Question

Ordering

Suppose we’re given a sequence S of n elements, each of which is colored red or blue. Assume S is represented as an array. Give an in-place method, Blue-Red(S, n), in a pseudo code for ordering S so that all the blue elements are listed before all the red elements.

Answer 1

In - place method = where input is overwritten by output during algorithm execution.

Let's consider,

1 represents all red coloured elements in the sequence and

0 represents all blue coloured elements in the sequence.

S = sequence array

n = size of array

// Pseudocode for ordering the sequence such that all blues are listed before all reds

Blue-Red( S, n )

{
countBlue = 0; // count number of elements representing blue colour i.e counting zeroes(0).

for ( index = 0; index < n; index++)

{ // count Blues in the sequence
if ( S[index] == 0 ) :

then countBlue++;

}

for ( index = 0; index < countBlue; index++)

{ // fill the sequence with 0 upto its count to represent blue elements in beginning

S[index] = 0;

}

for ( index = countBlue; index < n; index++)

{ // fill the sequence with 1 in remaining portion for red elements

S[index] = 1;

}

}

This way sequence S would contain blue coloured elements before the red ones.