Go: package main

import (
    "bufio"
    "os"
    "fmt"
)

var sc = bufio.NewScanner(os.Stdin)

/**************  Cell  *****************/
type Cell struct {
    item interface{}
    next *Cell
    prev *Cell
}

func NewCell(x interface{}, ne *Cell, pr *Cell) *Cell {
    newCp := new(Cell)
    newCp.item = x
    newCp.next = ne
    newCp.prev = pr
    return newCp
}
/************  End Cell  ***************/

/**************  Queue  ****************/
type Queue struct {
    size int
    front *Cell
    rear *Cell
}

func NewQueue() *Queue {
    return new(Queue)
}

func (q *Queue) Push(x interface{}) {
    newNode := NewCell(x, nil, nil)
    if q.size == 0 {
        q.front = newNode
        q.rear = newNode
    } else {
        newNode.prev = q.rear
        q.rear.next = newNode
        q.rear = newNode
    }
    q.size++
}

func (q *Queue) PopLeft() interface{} {
    if q.size == 0 {
        return nil
    } else {
        x := q.front.item
        if q.size == 1 {
            q.front = nil
            q.rear = nil
        } else {
            q.front = q.front.next
            q.front.prev = nil
        }
        q.size--
        return x
    }
}

func (q *Queue) IsEmpty() bool {
    return q.size == 0
}

func (q *Queue) Length() int {
    return q.size
}

type State struct {
    color string
    cnt int
}

func main() {
    for {
        sc.Scan()
        init := sc.Text()
        if init == "0" {
            break
        }
        bfs(init)
    }
}

func bfs(init string) {
    dic := make(map[string]int)
    que := NewQueue()
    que.Push(State{init, 0})
    length := len(init)
    isSolved := false
    for !que.IsEmpty() {
        state := que.PopLeft().(State)
        _, ok := dic[state.color]
        if ok {
            continue
        } else {
            dic[state.color] = state.cnt
        }

        if isSingleColor(state.color) {
            fmt.Println(state.cnt)
            isSolved = true
            break
        }

        for i := 0;i < length - 1; i++ {
            color1, color2 := string(state.color[i]), string(state.color[i + 1])
            if color1 != color2 {
                color3 := other(color1, color2)
                if i <= length - 2 {
                    que.Push(State{state.color[:i] + color3 + state.color[i + 2:], state.cnt + 1})
                } else {
                    que.Push(State{state.color[:i] + color3, state.cnt + 1})
                }
            }
        }
    }
    if !isSolved {
        fmt.Println("NA")
    }
}

func other(color1, color2 string) string {
    if color1 == "r" {
        if color2 == "g" {
            return "bb"
        } else if color2 == "b" {
            return "gg"
        }
    } else if color1 == "g" {
        if color2 == "r" {
            return "bb"
        } else if color2 == "b" {
            return "rr"
        }
    } else if color1 == "b" {
        if color2 == "r" {
            return "gg"
        } else if color2 == "g" {
            return "rr"
        }
    }
    return ""
}

func isSingleColor(color string) bool {
    for _, c := range color {
        if string(c) != string(color[0]) {
            return false
        }
    }
    return true
}

C++: #include <iostream>
#include <string>
#include <queue>
#include <vector>
#include <set>

using namespace std;

inline bool ok(const string & v){
    for (int i = 1; i < v.size(); i++){
        if (v[i] != v[0]) return false;
    }
    return true;
};

inline char other(char a, char b){
    if (a>b)swap(a, b);
    if (a == 'g' && b == 'r'){
        return 'b';
    }
    if (a == 'b' && b == 'g'){
        return 'r';
    }
    if (a == 'b' && b == 'r'){
        return 'g';
    }
    throw "";
}

struct state {
    string str;
    int n;
};

int main(){
    string s;
    while (cin >> s, s != "0"){
        queue<state> q;
        set<string> dp;
        q.push({ s, 0 });

        int ans = -1;
        while (q.size()){
            state sta = q.front();
            q.pop();
            string & sv = sta.str;
            auto pr = dp.insert(sv);
            if (!pr.second){
                continue;
            }
            if (ok(sv)){
                ans = sta.n;
                break;
            }
            for (int i = 1; i < sv.size(); i++){
                if (sv[i] != sv[i - 1]){
                    char a = sv[i];
                    char b = sv[i - 1];
                    sv[i] = sv[i - 1] = other(a, b);
                    q.push({ sv, sta.n + 1 });
                    sv[i] = a;
                    sv[i - 1] = b;
                }
            }
        }
        if (ans == -1){
            cout << "NA" << endl;
        }
        else{
            cout << ans << endl;
        }
    }
}

C++: #include <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
using namespace std;
#define INF 100000000
typedef long long ll;

int main(void) {
    while (1) {
        string s;
        cin >> s;
        if (s == "0") break;
        map<string, int> mi;
        queue<string> q;
        q.push(s);
        mi[s] = 0;
        bool flag = true;
        while (!q.empty()) {
            string now = q.front(); q.pop();
            // 全て同じ文字かどうかのチェック
            int i;
            for (i = 0; i < now.size()-1; i++) {
                if (now[i] != now[i+1]) {
                    flag = false;
                    break;
                }
            }
            if (i == now.size()-1) {
                cout << mi[now] << endl;
                break;
            }
            // いろいろ移行するところを考える
            for (int i = 0; i < now.size()-1; i++) {
                string s = now;
                if (s[i] != s[i+1]) {
                    if (s[i] != 'r' && s[i+1] != 'r') {
                        s[i] = 'r'; s[i+1] = 'r';
                    } else if (s[i] != 'g' && s[i+1] != 'g') {
                        s[i] = 'g'; s[i+1] = 'g';
                    } else {
                        s[i] = 'b'; s[i+1] = 'b';
                    }
                }
                if (mi.find(s) == mi.end()) {
                    q.push(s);
                    mi.insert(map<string, int>::value_type(s, mi[now]+1));
                }
            }
        }
        if (flag) cout << "NA" << endl;
    }
    return 0;
}