This code implement the Dutch Nation Flag problem, use three-way partition algorithms

dutch_nation_flag.go

package main

import (
	"fmt"
)

var flags = []int{1,2,3,3,2,1,1,2,3}

func main() {
	var i, j, n int

	var low = 1
	var high = 3

	i, j = 0, 0
	n = len(flags) - 1

	fmt.Printf("%#v\n", flags)

	for flags[j] == low {
		i++
		j++
	}

	for flags[n] == high {
		n--
	}

	//fmt.Printf("i=%d, j=%d, n=%d\n", i, j, n)
	for j <= n {
		switch flags[j] {
		case low:
			if i == j {
				j++
				continue
			}
			swap(i, j)
			i++
		case high:
			swap(n, j)
			n--
		default:
			j++
		}
		//fmt.Printf("i=%d, j=%d, n=%d\n", i, j, n)
	}
	fmt.Printf("%#v\n", flags)
}

func swap(x, y int) {
	//fmt.Printf("flags[%d]: %d <->flags[%d]: %d\n", x, flags[x], y, flags[y])
	flags[x], flags[y] = flags[y], flags[x]
}