The main function that links the list list to be traversed
function extractLeafList ( root ) { if ( root == null ) return null ; if ( root . left == null && root . right == null ) { if ( head == null ) { head = root ; prev = root ; } else { prev . right = root ; root . left = prev ; prev = root ; } return null ; } root . left = extractLeafList ( root . left ) ; root . right = extractLeafList ( root . right ) ; return root ; }