算法

merge-two-sorted-lists

解决办法： 递归

  public class Solution{
public ListNode mergeTwoLists(ListNode l1,ListNode l2){
    if(l1==null){
    return l2;
    }
    if(l2==null){
    return l1;
    }
    ListNode mergeHead;
    if(l1.val<l2.val){
    mergeHead=l1;
    mergeHead.next=mergeTwoLists(l1.next,l2);
    }else{
    mergeHead=l2;
    mergeHead.next=mergeTwoLists(l1,l2.next);
    }
    return mergeHead;
    }
}

解决方法：backtracking solution(回溯法)
此例运用递归回溯的方式

   public List<String> generateParenthesis(int n){
      List<String> list =new ArrayList<String> ();
      backtrack(list,"",0,0,n);
      return list;
}
public void backtrack(List<String> list,String str,int open,int close,int max){
    if(str.length()==max*2){
    list.add(str);
    }
    if(open<max)
    backtrack(list,str+"(",open+1,close,max);
    if(close<open)
    backtrack(list,str+")",open,close+1,max); 
}