make dictionary
let D = new Set ( ) ; D . add ( " " ) ; D . add ( " " ) ; D . add ( " " ) ; D . add ( " " ) ; D . add ( " " ) ; D . add ( " " ) ; D . add ( " " ) ; let start = " " ; let target = " " ; document . write ( " " + shortestChainLen ( start , target , D ) ) ;