How to arrange an array in decreasing order of frequency of each number?

Question

Input : {5, 13, 6, 5, 13, 7, 8, 6, 5}

Output : {5, 5, 5, 13, 13, 6, 6, 7, 8}

The question is to arrange the numbers in the array in

Answer 1

Sort the array, and in your sort function for x, y: sort by count(x) vs. count(y). If they are the same, sort by index(x) vs. index(y)

In python:

input = [5, 13, 6, 5, 13, 7, 8, 6, 5]
orig = list(input)

def cmp(x, y):
    if (orig.count(y) - orig.count(x) != 0):
        return orig.count(y) - orig.count(x)
    return orig.index(x) - orig.index(y)   

input.sort(cmp) 
print input

To make it more efficient, precalculate counts and indexes before sorting the array.