ts4: add all solutions chapter 3

Author: aikhelis

typescript/Fast and Slow Pointers/happy_number.ts

@@ -0,0 +1,27 @@
+function happyNumber(n: number): boolean {
+    let slow = n, fast = n;
+    while (true) {
+        slow = getNextNum(slow);
+        fast = getNextNum(getNextNum(fast));
+        if (fast == 1)
+            return true;
+        // If the fast and slow pointers meet, a cycle is detected. 
+        // Hence, 'n' is not a happy number.
+        else if (fast == slow)
+            return false;
+    }
+}
+
+function getNextNum(x: number): number {
+    let nextNum = 0;
+    while (x > 0){
+        // Extract the last digit of 'x'.
+        const digit = x % 10;
+        // Truncate (remove) the last digit from 'x' using floor 
+        // division.
+        x /= 10;
+        // Add the square of the extracted digit to the sum.
+        nextNum += digit ** 2;
+    }
+    return nextNum;
+}

typescript/Fast and Slow Pointers/linked_list_loop.ts

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+/* Definition of ListNode: 
+class ListNode {
+    val: number; next: ListNode | null;
+    constructor(val: number) {
+        this.val = val;
+        this.next = null;
+    }
+}    
+*/
+
+function linkedListLoop(head: ListNode): boolean {
+    let slow: ListNode | null = head;
+    let fast: ListNode | null = head;
+    // Check both 'fast' and 'fast.next' to avoid null pointer
+    // exceptions when we perform 'fast.next' and 'fast.next.next'.
+    while (fast !== null && fast.next !== null) {
+        slow = slow!.next;
+        fast = fast.next.next!;
+        if (fast == slow)
+            return true;
+    }
+    return false;
+}

typescript/Fast and Slow Pointers/linked_list_loop_naive.ts

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+/* Definition of ListNode:
+class ListNode {
+    val: number; next: ListNode | null;
+    constructor(val: number) {
+        this.val = val;
+        this.next = null;
+    }
+}    
+*/
+
+function linkedListLoopNaive(head: ListNode): boolean {
+    const visited: Set<ListNode> = new Set();
+    let curr: ListNode | null = head;
+    while (curr !== null) {
+        // Cycle detected if the current node has already been visited.
+        if (visited.has(curr))
+            return true;
+        visited.add(curr);
+        curr = curr.next;
+    }
+    return false;
+}

typescript/Fast and Slow Pointers/linked_list_midpoint.ts

@@ -0,0 +1,22 @@
+/* Definition of ListNode: 
+class ListNode {
+    val: number; next: ListNode | null;
+    constructor(val: number) {
+        this.val = val;
+        this.next = null;
+    }
+}    
+*/
+
+
+function linkedListMidpoint(head: ListNode): ListNode | null {
+    let slow: ListNode | null = head;
+    let fast: ListNode | null = head;
+    // When the fast pointer reaches the end of the list, the slow
+    // pointer will be at the midpoint of the linked list.
+    while (fast !== null && fast.next !== null) {
+        slow = slow!.next;
+        fast = fast.next.next!;
+    }
+    return slow;
+}