题目

方法

有序链表，合并成一个有序链表。

	   public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
	        if (l1 == null) {
	            return l2;
	        }
	        if (l2 == null) {
	            return l1;
	        }
	        
	        ListNode head = null;
	        ListNode end = null;
	        ListNode first = l1;
	        ListNode second = l2;
	        while (first != null && second != null) {
	            int temp1 = first.val;
	            int temp2 = second.val;
	            if (temp1 <= temp2) {
	                if (head == null) {
	                    head = first; 
	                    end = head;
	                } else {
	                    end.next = first;
	                    end = first;
	                }
	                first = first.next;
	            } else {
	                if (head == null) {
	                    head = second; 
	                    end = head;
	                } else {
	                    end.next = second;
	                    end = second;
	                }
	                second = second.next;
	            }
	            
	        }
	        if (first != null) {
	            end.next = first;
	        }
	        if (second != null) {
	            end.next = second;
	        }
	        return head;
	    }