Sort Colors II
Given an array of n objects with k different colors (numbered from
k), sort them so that objects of the same color are adjacent, with the colors in the order
1, 2, ... k.
You are not suppose to use the library's sort function for this problem.
colors = [3, 2, 2, 1, 4],
k = 4, your code should sort colors in-place to
[1, 2, 2, 3, 4].