Add reverse-linked-list solution

Author: Jeehay28

reverse-linked-list/Jeehay28.js

@@ -0,0 +1,82 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+// The algorithm uses a constant amount of extra space: prev, current, and nextTemp.
+// No additional data structures (like arrays or new linked lists) are created.
+// Hence, the space complexity is O(1).
+
+var reverseList = function (head) {
+  let prev = null;
+  let current = head;
+
+  while (current) {
+    let nextTemp = current.next;
+
+    current.next = prev;
+    console.log(current, prev);
+
+    prev = current;
+    console.log(current, prev);
+    current = nextTemp;
+  }
+
+  return prev; // New head of the reversed list
+};
+
+// head = [1,2,3,4,5]
+// [1] null
+// [1] [1]
+// [2,1] [1]
+// [2,1] [2,1]
+// [3,2,1] [2,1]
+// [3,2,1] [3,2,1]
+// [4,3,2,1] [3,2,1]
+// [4,3,2,1] [4,3,2,1]
+// [5,4,3,2,1] [5,4,3,2,1]
+
+// my own approach
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+var reverseList = function (head) {
+  if (head === null) {
+    return null;
+  }
+
+  let current = head;
+  let arr = [];
+  // console.log(head.val)
+
+  // traverse the linked List - TC: O(n)
+  while (current) {
+    arr.push(current.val);
+    current = current.next;
+  }
+
+  // reverse the array - TC: O(n)
+  arr = arr.reverse();
+
+  let head1 = new ListNode(arr[0]);
+  let current1 = head1;
+
+  // rebuild the linked list - TC: O(n)
+  for (let i = 1; i < arr.length; i++) {
+    current1.next = new ListNode(arr[i]);
+    current1 = current1.next;
+  }
+
+  return head1;
+};
+
+