递归实现

function mergeList(l1,l2){
    if(l1==null)return l2
    if(l2==null)return l1
    if(l1.val<=l2.val){
        l1.next=mergeList(l1.next,l2)
        return l1
    }else{
        l2.next=mergeList(l2.next,l1)
        return l2
    }
 
}