function solve(s){
return s.replace(/R\.+L/g,str=>{
let len=str.length
let halfLen=parseInt(len/2)
let middle=len%2!==0?"#":""
return 'R'.repeat(halfLen)+middle+'L'.repeat(halfLen)
})
.replace(/R\.+/g, str => 'R'.repeat(str.length))
.replace(/\.+L/g, str => 'L'.repeat(str.length))
.replace(/#/g, '.')
}
console.log(solve('RRRLLL.R')