SOURCE

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')