feat: add LeetCode problem 92

leetcode/src/92.c

@@ -0,0 +1,46 @@
+/*
+ * 92. Reverse Linked List II
+ * https://leetcode.com/problems/reverse-linked-list-ii/
+ *
+ * Given the head of a singly linked list and two integers left
+ * and right where left <= right, reverse the nodes of the list
+ * from position left to position right (1-indexed), and return
+ * the reversed list.
+ *
+ * Approach: use a dummy node before head to simplify edge cases
+ * (e.g. left == 1). Walk to the node just before position `left`,
+ * then reverse the next (right - left + 1) nodes in place using
+ * the same prev/curr pointer technique as a full list reversal,
+ * finally reconnecting the reversed segment to the rest of the list.
+ * Time: O(n), Space: O(1)
+ */
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+struct ListNode *reverseBetween(struct ListNode *head, int left, int right) {
+    struct ListNode dummy;
+    dummy.next = head;
+
+    struct ListNode *before = &dummy;
+    for (int i = 1; i < left; i++) {
+        before = before->next;
+    }
+
+    struct ListNode *curr = before->next;
+    struct ListNode *prev = NULL;
+
+    for (int i = 0; i <= right - left; i++) {
+        struct ListNode *next = curr->next;
+        curr->next = prev;
+        prev = curr;
+        curr = next;
+    }
+
+    before->next->next = curr;
+    before->next = prev;
+
+    return dummy.next;
+}