LC#21 merge two sorted linkedlists, 2 approaches, recursive and iterative

Author: NirmalSilwal

Leetcode/leetcodeTags/LinkedList/MergeTwoSortedLists21.java

@@ -0,0 +1,84 @@
+package LinkedList;
+
+public class MergeTwoSortedLists21 {
+
+	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;
+		}
+	}
+
+	// recursive
+	public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+
+		return merge(list1, list2);
+	}
+
+	private ListNode merge(ListNode list1, ListNode list2) {
+
+		if (list1 == null)
+			return list2;
+		if (list2 == null)
+			return list1;
+
+		if (list1.val < list2.val) {
+			list1.next = merge(list1.next, list2);
+			return list1;
+		} else {
+			list2.next = merge(list1, list2.next);
+			return list2;
+		}
+	}
+
+	// iterative
+	public ListNode mergeTwoListsIterative(ListNode list1, ListNode list2) {
+		if (list1 == null)
+			return list2;
+		if (list2 == null)
+			return list1;
+
+		ListNode mergedAns;
+		ListNode tail;
+
+		if (list1.val < list2.val) {
+			mergedAns = list1;
+			tail = list1;
+			list1 = list1.next;
+		} else {
+			mergedAns = list2;
+			tail = list2;
+			list2 = list2.next;
+		}
+
+		while (list1 != null && list2 != null) {
+			if (list1.val < list2.val) {
+				tail.next = list1;
+				tail = list1;
+				list1 = list1.next;
+			} else {
+				tail.next = list2;
+				tail = list2;
+				list2 = list2.next;
+			}
+		}
+
+		if (list1 == null)
+			tail.next = list2;
+
+		if (list2 == null)
+			tail.next = list1;
+
+		return mergedAns;
+	}
+}