The fourth hardest challenge listed in the “easy” section of Coderbyte.

correctPath.js

function isValid(string) {
  
  let v = 0 
  let h =  0
  
  for (let char of string) {
    
    if (char === 'd') v++
    if (char === 'u') v--
    if (char === 'r') h++
    if (char === 'l') h--
  }
  
  for (let i = 1; i < string.length; i++) {

    if (string[i - 1] + string[i] in {du: 1, ud: 1, rl: 1, lr: 1}) return false
  }
  
  return v === h && v === 4
}

function combination(length, symbols) {
  
  let combinations = [];
  
  (function recursion(s = '') {
    
    if (s.length === length) return combinations.push(s)
  
    for (let symbol of symbols) recursion(s + symbol)
  })()
  
  return combinations
}

function correctPath(string) {

  let countMarks = string.replace(/[^?]/g, '').length
  
  let combinations = combination(countMarks, 'durl')
  
  let startsWithMark = string.indexOf('?') === 0
  
  for (let combination of combinations) {
    
    combination = [...combination]
    let temp = string
    
    if (startsWithMark && combination[0] in {u: 1, l: 1}) continue
        
    while (temp.indexOf('?') > -1) {
      
      temp = temp.replace('?', v => {
        
        return combination.pop()
      })
    }
    
    if (isValid(temp)) return temp
  }
}