Print list of valid IPs from passed string

print_ips.go

package main

import (
	"fmt"
	"strconv"
)


type tree struct {
	nodes []*tree
	val   int
}

func (t *tree) buildTree(s string) {
	n := 3
	if len(s) < n {
		n = len(s)
	}

	for i := 1; i <= n; i++ {
		x, _ := strconv.Atoi(string(s[0:i]))
		if x < 256 {
			node := tree{make([]*tree, 3), x}
			node.buildTree(string(s[i:]))
			t.nodes[i-1] = &node
		}
	}
}


func printIP(t *tree) {
	fmt.Println("List of valid IPs")
	var walk func(*tree, int, string)
	walk = func(t *tree, level int, prefix string) {
		if level > 4 {
			return
		}
		if t.nodes[0] == nil && t.nodes[1] == nil && t.nodes[2] == nil && level == 4 {
			fmt.Println(prefix + "." + strconv.Itoa(t.val))
		}
		for i := 0; i < 3; i++ {
			if t.nodes[i] != nil {
				var s string
				if level > 0 {
					if prefix != "" {
						s = prefix + "." + strconv.Itoa(t.val)
					} else {
						s = strconv.Itoa(t.val)
					}
				}
				walk(t.nodes[i], level+1, s)
			}
		}
	}
	walk(t, 0, "")
}

func main() {
	var s string
	fmt.Scanln(&s)
	// s := "12323201"  // uncomment to try hard coded string, without reading from stdio
	
	if len(s) < 4 || len(s) > 12 {
		return
	}

	r := tree{make([]*tree, 3), -1}

	r.buildTree(s)
	printIP(&r)
}