融合链表

package com.algorithm;

public class MergeList {
      public ListNode Merge(ListNode list1,ListNode list2) {
          //运行时间：32ms
          //占用内存：688k
          //输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。
          //第一次比较
          if(list1 == null) 
              return list2;
          if(list2 == null) 
              return list1;
          ListNode listGuard = null;//哨兵，存储链表头指针
          ListNode listroot = null;
          //然后每次比较两条链表的每一个结点的大小，如果小的，那么将哨兵的next指向它，再
          //将小的节点指向next
          if(list1.val <= list2.val) {
              listroot = list1;
              listGuard = list1;
              list1 = list1.next;
          }else{
              listroot = list2;
              listGuard = list2;
              list2 = list2.next;
          }
          while(list1 != null && list2 != null){
              if(list1.val <= list2.val){//如果第一条链表小于第二条链表
                  listGuard.next = list1;
                  listGuard = list1;
                  list1 = list1.next;
              }else{
                  listGuard.next = list2;
                  listGuard = list2;
                  list2 = list2.next;
              }
          }
          //循环遍历完了，如果第一个不等于空，则说明后面还有数据，直接移动指针listGuard即可
          if(list1 != null) {
              listGuard.next = list1;42           }
          if(list2 != null) {
              listGuard.next = list2;47           }
          return listroot;
        }
      public static void main(String[] args) {
             ListNode la = new ListNode(1);
              ListNode lb = new ListNode(3);
              ListNode lc = new ListNode(9);
              la.next = lb;
              lb.next = lc;
              lc.next = null;
              ListNode ma = new ListNode(4);
              ListNode mb = new ListNode(6);
              ListNode mc = new ListNode(7);
              ma.next = mb;
              mb.next = mc;
              mc.next = null;
              ListNode root = new MergeList().Merge(la,ma);
              while(root!=null){
                  System.out.print(root.val + " ");
                  root = root.next;
              }
        }
}

public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}

  //递归
        if(list1 == null)
        return list2;
       if(list2 == null)
        return list1;
       if(list1.val <= list2.val){
            list1.next = Merge(list1.next, list2);
            return list1;
        }else{
            list2.next = Merge(list1, list2.next);
            return list2;
        }