Code that solves the problem of organizing an array (I used a golang slice) with a random distribution of red, green and blue values, into red values followed by blue values followed by green values, in O(n) time.

rgb.go

//
//  Written By Joel Simonoff
//  April 24, 2017
//

package main

import (
	"fmt"
	"math/rand"
	"time"
)

func main() {
	failCount := 0
	numRuns := 100
	numVals := 67

	for i := 0; i < numRuns; i++ {
		s := generateData(numVals)
		fmt.Printf("Run 1\nBefore: %v\n\n", s)
		rgb(&s)
		if validateResult(s) {
			fmt.Printf("After: %v\nTest Passed!\n\n", s)
		} else {
			failCount++
			fmt.Printf("TEST FAILED 😐 After:%v\n\n", s)
		}
	}

	fmt.Printf("Summary\nNum Fails: %v,\nNum Runs: %v,\nNum Vals Per Run: %v\n", failCount, numRuns, numVals)
}

//
// r = 0, g = 1, b = 2
//
func rgb(p *([]int)) {
	s := *p
	g := -1
	b := -1

	for i := range s {
		if s[i] == 2 {
			handleBlue(s, i, &b)
			continue
		}
		if s[i] == 1 {
			handleGreen(s, i, &b, &g)
			continue
		}

		handleRed(s, i, &b, &g)
	}

	*p = s
}

func handleBlue(s []int, i int, b *int) {
	checkNotSetAndReplace(b, i)
}

func handleGreen(s []int, i int, b, g *int) {
	if *b != -1 {
		swap(s, i, *b)

		*b++

		checkNotSetAndReplace(g, *b-1)

		return
	}

	checkNotSetAndReplace(g, i)
}

func handleRed(s []int, i int, b, g *int) {
	if *g != -1 {
		swap(s, i, *g)

		*g++
	}

	if *b != -1 {
		swap(s, i, *b)

		*b++
	}
}

func validateResult(s []int) bool {
	for i := 0; i < len(s)-1; i++ {
		if s[i] > s[i+1] {
			return false
		}
	}

	return true
}

// checkNotSetAndReplace replaces the value at location v with i if *v == -1.
func checkNotSetAndReplace(v *int, i int) {
	if *v == -1 {
		*v = i
	}
}

func swap(s []int, l int, r int) {
	s[l], s[r] = s[r], s[l]
}

func generateData(n int) []int {
	rand.Seed(time.Now().UnixNano())

	s := make([]int, n)

	for i := 0; i < n; i++ {
		s[i] = rand.Intn(3)
	}

	return s
}

func printSlice(s []int) {
	for _, i := range s {
		fmt.Print(i)
	}
}