LC#234 palindrom LinkedList, 2 approaches, O(N) Time, O(1) & O(N) Space

Author: NirmalSilwal

Leetcode/easy/PalindromeLinkedList234.java

@@ -0,0 +1,87 @@
+package easy;
+
+import java.util.ArrayList;
+
+public class PalindromeLinkedList234 {
+
+	private class ListNode {
+		int val;
+		ListNode next;
+
+		ListNode() {
+		}
+
+		ListNode(int val) {
+			this.val = val;
+		}
+
+		ListNode(int val, ListNode next) {
+			this.val = val;
+			this.next = next;
+		}
+	}
+
+	public boolean isPalindrome1(ListNode head) {
+
+		ArrayList<Integer> list = new ArrayList<>();
+		ListNode temp = head;
+
+		while (temp != null) {
+			list.add(temp.val);
+			temp = temp.next;
+		}
+
+		int front = 0, back = list.size() - 1;
+
+		while (front <= back) {
+			if (list.get(front) != list.get(back)) {
+				return false;
+			}
+			front++;
+			back--;
+		}
+
+		return true;
+	}
+
+	// 2nd approach
+	// O(N) Time | O(1) Space
+	public static boolean isPalindrome(ListNode head) {
+
+		ListNode slow = head;
+		ListNode fast = head;
+
+		// by the end of this loop, fast will reach end of List & slow to mid of
+		// List
+		while (fast.next != null && fast.next.next != null) {
+			fast = fast.next.next;
+			slow = slow.next;
+		}
+		// reversing the right half of the List
+		ListNode secondHalfHead = reversed(slow.next);
+		ListNode firstHalfHead = head;
+
+		// traversing two heads & comparing values
+		while (secondHalfHead != null && firstHalfHead != null) {
+			if (secondHalfHead.val != firstHalfHead.val) {
+				return false;
+			}
+
+			secondHalfHead = secondHalfHead.next;
+			firstHalfHead = firstHalfHead.next;
+		}
+		return true;
+	}
+
+	private static ListNode reversed(ListNode head) {
+		ListNode newHead = null;
+		while (head != null) {
+			ListNode next = head.next;
+			head.next = newHead;
+			newHead = head;
+			head = next;
+		}
+		return newHead;
+	}
+
+}