LintCode - Sort Colors II

Given an array of n objects with k different colors (numbered from 1 to k), sort them so that objects of the same color are adjacent, with the colors in the order 1, 2, ... k.

 

Have you met this question in a real interview? 

Yes

Example

Given colors=[3, 2, 2, 1, 4], k=4, your code should sort colors in-place to [1, 2, 2, 3, 4].

Note

You are not suppose to use the library's sort function for this problem.

Challenge

A rather straight forward solution is a two-pass algorithm using counting sort. That will cost O(k) extra memory. Can you do it without using extra memory?

Solution:

public void sortColors2(int[] colors, int k) {
    int n = colors.length;
    int[] B = new int[n];
    int[] C = new int[k+1];
    for(int a:colors) C[a]++;
    for(int i=1; i<=k; i++) {
        C[i] += C[i-1];
    }
    for(int a:colors) {
        C[a]--;
        B[C[a]] = a;
    }
    System.arraycopy(B, 0, colors, 0, n);
}

Reference:

https://zh.wikipedia.org/wiki/计数排序

http://www.geeksforgeeks.org/counting-sort/

https://www.cs.usfca.edu/~galles/visualization/CountingSort.html