Reverse Singly Linked List

LeetCode/ReverseLinkedList_LC206.java

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+public class ReverseLinkedList_LC206 {
+    public ListNode reverseList(ListNode head) {
+        // return reverseIterative(head);
+        return reverseRecursive(head);
+    }
+
+    /**
+     * Keep three pointers at a node, it's previous, itself and its next. Iterate until end/null is reached
+     * In every iteration, save copy of next, make current's next point to previous and update previous and current
+     *
+     * Time Complexity: O(N) N:number of nodes
+     * Space Complexity: O(1)
+     *
+     * @param node
+     * @return
+     */
+    ListNode reverseIterative(ListNode node) {
+        ListNode previous = null, current = node, temp = null;
+        while (current != null) {
+            temp = current.next;
+            current.next = previous;
+            previous = current;
+            current = temp;
+        }
+        return previous;
+    }
+
+    /**
+     * With recursive function, reach all the way down to last node which would be head for reversedlist
+     * Once end is reached, keep returning and update current node such that returned and reversed nodes point to
+     * the current node. Change pointers while returning from each recursive call
+     *
+     * Time Complexity: O(N) N:number of nodes
+     * Space Complexity: O(N) Stack space
+     *
+     * @param node
+     * @return
+     */
+    ListNode reverseRecursive(ListNode node) {
+        if (node == null || node.next == null)
+            return node;
+
+        ListNode head = reverseRecursive(node.next);
+        node.next.next = node;
+        node.next = null;
+        return head;
+    }
+}
+
+class ListNode {
+    int val;
+    ListNode next;
+
+    ListNode() {
+    }
+
+    ListNode(int val) {
+        this.val = val;
+    }
+
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+}