Problem

Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note: You are not suppose to use the library’s sort function for this problem.

example 1

1
2

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Follow up:

• A rather straight forward solution is a two-pass algorithm using counting sort. First, iterate the array counting number of 0’s, 1’s, and 2’s, then overwrite array with total number of 0’s, then 1’s and followed by 2’s.
• Could you come up with a one-pass algorithm using only constant space?

Solution

In place sort

Main idea is use two pointers: end of zero sequence, and end of one sequene.

Walk throw array and check if current element in zero – swap it with next element of end of zero sequence. If current element is one – swap in with next element of end of one sequene.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18

func sortColors(nums []int)  {
    ez:=-1
    eo:=-1
    
    for idx:=range nums {
        if nums[idx] == 0 && ez != idx {
            ez++
            nums[idx], nums[ez] = nums[ez], nums[idx]
        }
        if nums[idx] == 1 && eo != idx {
            if eo == -1 {
                eo = ez
            }
            eo++
            nums[idx], nums[eo] = nums[eo], nums[idx]
        }
    }
}