Add a Solution for Merging Two Sorted Linked Lists (LeetCode Problem 21)

dsa-solutions/lc-solutions/0000-0099/0021-Merge-two-sorted-lists.md

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+---
+id: merge-two-sorted-lists
+title: Merge Two Sorted Lists (LeetCode)
+sidebar_label: 0021-MergeTwoSortedLists
+tags:
+  - Linked List
+  - Two Pointers
+description: Given two sorted linked lists, merge them into a single sorted linked list.
+---
+
+## Problem Description
+
+| Problem Statement | Solution Link | LeetCode Profile |
+| :---------------- | :------------ | :--------------- |
+| [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/) | [Merge Two Sorted Lists Solution on LeetCode](https://leetcode.com/problems/merge-two-sorted-lists/solutions/) |  [VijayShankerSharma](https://leetcode.com/u/darkknight648/) |
+
+### Problem Description
+
+Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.
+
+### Examples
+
+#### Example 1
+
+- **Input:** list1 = [1,2,4], list2 = [1,3,4]
+- **Output:** [1,1,2,3,4,4]
+
+#### Example 2
+
+- **Input:** list1 = [], list2 = []
+- **Output:** []
+
+#### Example 3
+
+- **Input:** list1 = [], list2 = [0]
+- **Output:** [0]
+
+### Constraints
+
+- 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.
+
+### Approach
+
+To solve the problem, we can use the following approach:
+
+1. **Initialize Pointers:**
+   - Use a dummy node to form the new list and a tail pointer to keep track of the end of the merged list.
+
+2. **Iterate Through the Lists:**
+   - Compare the values of the current nodes in both lists and attach the smaller node to the merged list.
+   - Move the pointer forward in the list from which the node was taken.
+
+3. **Attach Remaining Nodes:**
+   - Once one of the lists is exhausted, attach the remaining nodes of the other list to the merged list.
+
+4. **Return Result:**
+   - The merged list is pointed to by the dummy node's next pointer.
+
+### Solution Code
+
+#### Python
+
+```python
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+class Solution:
+    def mergeTwoLists(self, list1, list2):
+        # Dummy node to form the new list
+        dummy = ListNode(0)
+        # Tail pointer for the new list
+        tail = dummy
+
+        # While both lists are non-empty, compare the values and attach the smaller node to the new list
+        while list1 is not None and list2 is not None:
+            if list1.val <= list2.val:
+                tail.next = list1
+                list1 = list1.next
+            else:
+                tail.next = list2
+                list2 = list2.next
+            tail = tail.next
+
+        # If either list is not empty, attach the remainder to the new list
+        if list1 is not None:
+            tail.next = list1
+        else:
+            tail.next = list2
+
+        # The dummy node's next pointer points to the head of the merged list
+        return dummy.next
+```
+
+#### C++
+
+```cpp
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
+        // Dummy node to form the new list
+        ListNode dummy(0);
+        // Tail pointer for the new list
+        ListNode* tail = &dummy;
+
+        // While both lists are non-empty, compare the values and attach the smaller node to the new list
+        while (list1 != nullptr && list2 != nullptr) {
+            if (list1->val <= list2->val) {
+                tail->next = list1;
+                list1 = list1->next;
+            } else {
+                tail->next = list2;
+                list2 = list2->next;
+            }
+            tail = tail->next;
+        }
+
+        // If either list is not empty, attach the remainder to the new list
+        if (list1 != nullptr) {
+            tail->next = list1;
+        } else {
+            tail->next = list2;
+        }
+
+        // The dummy node's next pointer points to the head of the merged list
+        return dummy.next;
+    }
+};
+```
+
+#### Java
+
+```java
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        // Dummy node to form the new list
+        ListNode dummy = new ListNode(0);
+        // Tail pointer for the new list
+        ListNode tail = dummy;
+
+        // While both lists are non-empty, compare the values and attach the smaller node to the new list
+        while (list1 != null && list2 != null) {
+            if (list1.val <= list2.val) {
+                tail.next = list1;
+                list1 = list1.next;
+            } else {
+                tail.next = list2;
+                list2 = list2.next;
+            }
+            tail = tail.next;
+        }
+
+        // If either list is not empty, attach the remainder to the new list
+        if (list1 != null) {
+            tail.next = list1;
+        } else {
+            tail.next = list2;
+        }
+
+        // The dummy node's next pointer points to the head of the merged list
+        return dummy.next;
+    }
+}
+```
+
+### Conclusion
+
+The above solution efficiently merges two sorted linked lists into a single sorted linked list. It employs a straightforward approach to compare the elements from both lists and attach the smaller element to the merged list. This ensures that the merged list maintains the sorted order of the input lists, providing a simple yet effective approach to solving the problem of merging two sorted linked lists.
+
+