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
}

Python: from collections import deque
import copy

def another_color(a, b):
    if not a == 'r' and not b == 'r':
        return 'r'
    if not a == 'g' and not b == 'g':
        return 'g'
    return 'b'


def is_all_same_elements(es):
    return all([e == es[0] for e in es[1:]]) if es else False

class State:
    def __init__(self, sequence):
        self.colors = list(sequence)
        self.label = sequence
        self.depth = 0

    def nextStates(self):
        nextStates = []
        for i in range(1, len(self.colors)):
            if self.colors[i] != self.colors[i - 1]:
                c = another_color(self.colors[i], self.colors[i - 1])
                newColors = copy.copy(self.colors)
                newColors[i] = c
                newColors[i - 1] = c
                nextStates.append(State(''.join(newColors)))
        return nextStates

    def show(self):
        print(self.depth)
        # for debug
#         print("{} {}".format(self.label, self.depth))


sequence = input()
while sequence != '0':
    root = State(sequence)
    printed = False
    visited = [root.label]
    queue = deque([])
    queue.append(root)
    # BFS
    while queue:
        state = queue.popleft()
        if is_all_same_elements(state.colors):
            state.show()
            printed = True
            break
        for nextState in state.nextStates():
            if not nextState.label in visited:
                visited.append(nextState.label)
                nextState.depth = state.depth + 1
                queue.append(nextState)

    if not printed:
        print("NA")
    # 次の行へ
    sequence = input()

Python: from collections import deque

def new_color(s, i, color):
  return s[:i - 1] + color * 2 + s[i + 1:]

def solve(s):
  length = len(s)
  monos = ["r" * length, "g" * length, "b" * length]
  if s in monos:
    print(0)
    return

  another = {("r", "g"):"b", ("g", "r"):"b", 
             ("r", "b"):"g", ("b", "r"):"g",
             ("g", "b"):"r", ("b", "g"):"r"}
  visited = {s:True}
  que = deque()
  app = que.append
  pop = que.popleft
  que.append((s, 0))

  while que:
    colors, score = pop()
    score += 1
    temp = colors[0]

    for i in range(1, length):
      ci = colors[i]
      if ci != temp:
        new = new_color(colors, i, another[(ci, temp)])
        if new in monos:
          print(score)
          return

        if new not in visited:
          visited[new] = True
          app((new, score))

      temp = ci

  print("NA")

def main():
  while True:
    s = input()
    if s == "0":
      break
    solve(s)

main()

Python: from collections import deque

def new_color(s, i, color):
  return s[:i] + color * 2 + s[i + 2:]

def solve(s):
  length = len(s)
  monos = ["r" * length, "g" * length, "b" * length]
  if s in monos:
    print(0)
    return
  rgb = "rgb"
  another = {("r", "g"):"b", ("g", "r"):"b", 
             ("r", "b"):"g", ("b", "r"):"g",
             ("g", "b"):"r", ("b", "g"):"r"}
  dic = {s:0}
  que = deque()
  app = que.append
  pop = que.popleft
  que.append((s, 0))

  while que:
    colors, score = pop()
    score += 1
    temp = colors[0]

    for i in range(1, length):
      ci = colors[i]
      if ci != temp:
        new = new_color(colors, i - 1, another[(ci, temp)])

        if new in monos:
          print(score)
          return

        if new not in dic:
          dic[new] = score
          app((new, score))

      temp = ci

  print("NA")

def main():
  while True:
    s = input()
    if s == "0":
      break
    solve(s)

main()