index.js

function decorateTree(base) {
  let combinations = {
    'PP': 'P', 'RR': 'R', 'BB': 'B',
    'PB': 'R', 'BP': 'R', 'RP': 'B',
    'PR': 'B', 'BR': 'P', 'RB': 'P'
  }

  const result = [ base ]
  let baseToArray = base.split(' ')
  let list = new Array(baseToArray.length - 1).fill('')

  list.map(_ => {
    baseToArray = [ ...baseToArray ].splice(1).map((val, index) => {
      const key = baseToArray[index] + val
      const resultCombination = combinations[key]
      return resultCombination
    })

    result.push(baseToArray.join(' '))
  })

  return result.reverse()
}

function decorateTreeAlt(base) {
  const dict = {
    "PP": "P",
    "BB": "B",
    "RR": "R",
    "BP": "R",
    "PB": "R",
    "BR": "P",
    "RB": "P",
    "PR": "B",
    "RP": "B"
  }

  base = base.split(" ")
  let list = new Array(base.length).fill(base)
  
  return list.reduce((total, x) =>
    total.concat(
      [ new Array(total.at(-1).length - 1).fill("-").map((_, i) => {
        return dict[total.at(-1).slice(i, i + 2).join("")]
      }) ]
    ), [base]
  ).slice(0, base.length).map(x => x.join(" ")).reverse()
}

module.exports = decorateTree