code1:(常规解法)
public ListNode mergeTwoLists(ListNode l1, ListNode l2) { if(l1==null) return l2; if(l2==null) return l1; ListNode head=null; ListNode node=null; while(l1!=null&&l2!=null){ if(l1.val>l2.val){ if(head==null){ head=l2; node=l2; l2=l2.next; } else{ node.next=l2; l2=l2.next; node=node.next; } }else{ if(head==null){ head=l1; node=l1; l1=l1.next; } else{ node.next=l1; l1=l1.next; node=node.next; } } } if(l1==null) node.next=l2; else node.next=l1; return head; }
code2:(优化后的递归解法)
public ListNode mergeTwoLists(ListNode l1, ListNode l2) { if (l1 == null) return l2; if (l2 == null) return l1; if (l1.val < l2.val) { l1.next = mergeTwoLists(l1.next, l2); return l1; } else { l2.next = mergeTwoLists(l1, l2.next); return l2; } }