21. Merge Two Sorted Lists

정리 코드

Recursion Function(재귀함수) : 자기 자신을 재참조, 종료 조건으로 종료 시점을 제어한다.

leetcode : 21. Merge Two Sorted Lists

• 조건
  • The number of nodes in both lists is in the range [0, 50].
    
  • -100 <= Node.val <= 100
    
  • Both list1 and list2 are sorted in non-decreasing order.
    

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode answer = new ListNode();
        if (list1 == null && list2 == null) return null;
        mergeTwoListsHelper(list1, list2, answer);
        return answer.next;
    }

    public void mergeTwoListsHelper(ListNode list1, ListNode list2, ListNode answer) {
        ListNode current = answer;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                current.next = list1;
                list1 = list1.next;
            } else {
                current.next = list2;
                list2 = list2.next;
            }
            current = current.next;
        }
        if (list1 != null) {
            current.next = list1;
        } else {
            current.next = list2;
        }
    }
}