update 206 java

yennanliu · May 25, 2024 · 20f1677 · 20f1677
1 parent 875eb57
commit 20f1677
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diff --git a/leetcode_java/src/main/java/LeetCodeJava/LinkedList/ReverseLinkedList.java b/leetcode_java/src/main/java/LeetCodeJava/LinkedList/ReverseLinkedList.java
@@ -24,13 +24,22 @@ public ListNode reverseList(ListNode head) {
                  *
                  *   4 operations
                  *
+                 *    Step 0) set _prev as null
                  *    step 1) cache next
                  *    step 2) point cur to prev
-                 *    step 3) move prev to cur
+                 *    step 3) move prev to cur  (NOTE !!! the step)
                  *    step 4) move cur to next
                  */
                 ListNode _next = head.next;
                 head.next = _prev;
+                /** NOTE !!!
+                 *
+                 *  -> have to assign _prev val to head first,
+                 *     then assign head val to _next,
+                 *     since if we assign head val to _next first,
+                 *     then head is changed (become "_next" node), then we will assign _prev to _next node,
+                 *     which is WRONG
+                 */
                 _prev = head;
                 head = _next;
             }

diff --git a/leetcode_java/src/main/java/dev/workspace3.java b/leetcode_java/src/main/java/dev/workspace3.java
@@ -1839,5 +1839,46 @@ public int getSum(List<Integer> cur) {
         return res;
     }
 
+    // LC 206
+    public ListNode reverseList_(ListNode head) {
+
+        if (head == null) {
+            return null;
+        }
+
+        ListNode root = new ListNode();
+        ListNode _prev = null;
+        root.next = _prev;
+
+        while (head != null) {
+            /**
+             *  NOTE !!!!
+             *
+             *   4 operations
+             *
+             *    Step 0) set _prev as null
+             *    step 1) cache next
+             *    step 2) point cur to prev
+             *    step 3) move prev to cur  (NOTE !!! the step)
+             *    step 4) move cur to next
+             */
+            ListNode _next = head.next;
+            head.next = _prev;
+            /** NOTE !!!
+             *
+             *  -> have to assign _prev val to head first,
+             *     then assign head val to _next,
+             *     since if we assign head val to _next first,
+             *     then head is changed (become "_next" node), then we will assign _prev to _next node,
+             *     which is WRONG
+             */
+            _prev = head;
+            head = _next;
+        }
+
+        // NOTE!!! we return _prev here, since it's now "new head"
+        return root.next; //_prev;
+    }
+
 
 }