merge two sorted lists solution

Author: wad-jangjaejeong

merge-two-sorted-lists/jaejeong1.java

@@ -0,0 +1,71 @@
+import java.util.List;
+
+//  Definition for singly-linked list.
+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 static void main(String[] args) {
+    Solution s = new Solution();
+    var list1 = new ListNode(1);
+    list1.next = new ListNode(2);
+    list1.next.next = new ListNode(4);
+
+    var list2 = new ListNode(1);
+    list2.next = new ListNode(3);
+    list2.next.next = new ListNode(4);
+
+    System.out.println(s.mergeTwoLists(list1, list2));
+  }
+
+  public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+    // A와 B 헤드를 비교
+    // A가 더 작거나 같으면 A 헤드를 빼서 새 노드로 추가
+    // 그렇지 않으면 B 헤드를 빼서 새 노드로 추가
+    // TC: O(N+M), N: list1의 길이, M: list2의 길이
+    // SC: O(N+M), N: list1의 길이, M: list2의 길이
+    ListNode mergedList = null;
+
+    while(list1 != null && list2 != null) {
+      if (list1.val <= list2.val) {
+        mergedList = addNode(mergedList, list1.val);
+        list1 = list1.next;
+      } else {
+        mergedList = addNode(mergedList, list2.val);
+        list2 = list2.next;
+      }
+    }
+
+    while(list1 != null) {
+      mergedList = addNode(mergedList, list1.val);
+      list1 = list1.next;
+    }
+
+    while(list2 != null) {
+      mergedList = addNode(mergedList, list2.val);
+      list2 = list2.next;
+    }
+
+    return mergedList;
+  }
+
+  private ListNode addNode(ListNode node, int val) {
+    if (node == null) {
+      node = new ListNode(val);
+    } else {
+      var last = node;
+      while(last.next != null) {
+        last = last.next;
+      }
+      last.next = new ListNode(val);
+    }
+
+    return node;
+  }
+}