feat: solve merge two sorted lists

Author: GangBean

merge-two-sorted-lists/GangBean.java

@@ -0,0 +1,73 @@
+/**
+ * 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) {
+        /**
+        1. understanding
+        - merge 2 sorted linked list
+        2. strategy
+        - assign return ListNode
+        - for each node, started in head, compare each node's value, and add smaller value node to return node, and move the node's head to head.next
+        3. complexity
+        - time: O(N + M), N is the length of list1, M is the length of list2
+        - space: O(1), exclude the return variable 
+        */
+        ListNode curr = null;
+        ListNode ret = null;
+
+        while (list1 != null && list2 != null) {
+            if (list1.val < list2.val) {
+                ListNode node = new ListNode(list1.val);
+                if (ret == null) {
+                    ret = node;
+                } else {
+                    curr.next = node;
+                }
+                list1 = list1.next;
+                curr = node;
+            } else {
+                ListNode node = new ListNode(list2.val);
+                if (ret == null) {
+                    ret = node;
+                } else {
+                    curr.next = node;
+                }
+                list2 = list2.next;
+                curr = node;
+            }
+        }
+
+        while (list1 != null) {
+            ListNode node = new ListNode(list1.val);
+            if (ret == null) {
+                ret = node;
+            } else {
+                curr.next = node;
+            }
+            list1 = list1.next;
+            curr = node;
+        }
+
+        while (list2 != null) {
+            ListNode node = new ListNode(list2.val);
+            if (ret == null) {
+                ret = node;
+            } else {
+                curr.next = node;
+            }
+            list2 = list2.next;
+            curr = node;
+        }
+
+        return ret;
+    }
+}
+