add solution of problem 234: palindrome linked list

Author: ppulse

PalindromeLinkedList234/README.md

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+Given a singly linked list, determine if it is a palindrome.
+
+####Follow up:
+Could you do it in O(n) time and O(1) space?

PalindromeLinkedList234/Solution.java

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+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+public class Solution {
+    public boolean isPalindrome(ListNode head) {
+        if (head == null)
+            return true;
+        
+        Stack<Integer> stack = new Stack<Integer>();
+        
+        ListNode currNode = head;
+        int len = 0;
+        
+        while (currNode != null) {
+            stack.push(currNode.val);
+            currNode = currNode.next;
+            len++;
+        }
+        
+        currNode = head;
+        int halfLen = len >> 1;
+        for (int i = 0; i <= halfLen; i++) {
+            if (currNode.val != stack.pop()) {
+                return false;
+            }
+            
+            currNode = currNode.next;
+        }
+        
+        return true;
+    }
+}