Add week 7 solutions: reverse-linked-list

Author: gitsunmin

reverse-linked-list/gitsunmin.ts

@@ -0,0 +1,50 @@
+/**
+ * https://leetcode.com/problems/reverse-linked-list
+ * time complexity : O(n)
+ * space complexity : O(1)
+ */
+
+class ListNode {
+    val: number
+    next: ListNode | null
+    constructor(val?: number, next?: ListNode | null) {
+        this.val = (val === undefined ? 0 : val)
+        this.next = (next === undefined ? null : next)
+    }
+}
+
+/** Not used in real problem */
+const arrayToListNode = (arr: number[]): ListNode | null => {
+    if (arr.length === 0) return null;
+    let head = new ListNode(arr[0]);
+    let curr = head;
+    for (let i = 1; i < arr.length; i++) {
+        curr.next = new ListNode(arr[i]);
+        curr = curr.next;
+    }
+    return head;
+}
+
+/* Main function */
+function reverseList(head: ListNode | null): ListNode | null {
+    let prev: ListNode | null = null;
+    let current = head;
+
+    while (current !== null) {
+        const next = current.next;
+        current.next = prev;
+        prev = current;
+        current = next;
+    }
+
+    return prev;
+};
+
+/* Examples */
+const input1 = [1, 2, 3, 4, 5];
+const input2 = [1, 2];
+const input3 = [];
+
+console.log('output1:', reverseList(arrayToListNode(input1)));
+console.log('output2:', reverseList(arrayToListNode(input2)));
+console.log('output3:', reverseList(arrayToListNode(input3)));