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Author: EndaLin

solution/0021.Merge Two Sorted Lists/Solution.java

@@ -1,16 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
 class Solution {
     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
-        if (l1 == null) {
-            return l2;
+        ListNode head = new ListNode(0);
+        head.next = null;
+        ListNode current = head;
+        while (l1 != null && l2 != null) {
+            if (l1.val <= l2.val) {
+                current.next = l1;
+                l1 = l1.next;
+            } else {
+                current.next = l2;
+                l2 = l2.next;
+            }
+            current = current.next;
         }
-        if (l2 == null) {
-            return l1;
-        }
-        if (l1.val < l2.val) {
-            l1.next = mergeTwoLists(l1.next, l2);
-            return l1;
-        }
-        l2.next = mergeTwoLists(l1, l2.next);
-        return l2;
+        current.next = l1 != null ? l1 : l2;
+        return head.next;
     }
 }

solution/0206.Reverse Linked List/Solution.java

@@ -0,0 +1,26 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        if (head == null) return null;
+        ListNode reverse = null;
+        while (head != null) {
+            ListNode temp = head;
+            head = head.next;
+            if (reverse == null) {
+                reverse = temp;
+                temp.next = null;
+            } else {
+                temp.next = reverse;
+                reverse = temp;
+            }
+        }
+        return reverse;
+    }
+}

solution/0876.Middle of the Linked List/Solution.java

@@ -1,18 +1,8 @@
-class Solution {
-    public ListNode middleNode(ListNode head) {
-        if (head == null || head.next == null) {
-            return head;
-        }
-        ListNode fast = head;
-        ListNode slow = head;
-        while (fast.next != null) {
-            // 快指针每次循环走两步，慢指针走一步
-            fast = fast.next.next;
-            slow = slow.next;
-            if (fast == null || fast.next == null) {
-                return slow;
-            }
-        }
-        return  null;
+public ListNode middleNode(ListNode head) {
+    ListNode low = head, first = head;
+    while (first != null && first.next != null) {
+        low = low.next;
+        first = first.next.next;
     }
+    return low;
 }