21. 合并两个有序链表

截屏2022-07-08 09.42.34.png

func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
        
        var temp1 = list1
        var temp2 = list2
        //添加虚拟头节点
        var end :ListNode? = ListNode()
        let ans = end
        while temp1 != nil && temp2 != nil {
            
            let n1 = temp1?.val ?? 0
            let n2 = temp2?.val ?? 0
            if n1 <= n2 {
                end?.next = temp1
                temp1 = temp1?.next
            }else {
                end?.next = temp2
                temp2 = temp2?.next
            }
            end = end?.next ?? nil
        }
        //需要补个尾
        end?.next = temp2 == nil ? temp1 : temp2
        
        
        return ans?.next
    
    }