add reorder list solution

Author: Tessa1217

reorder-list/Tessa1217.java

@@ -0,0 +1,59 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    // 시간복잡도: O(n), 공간복잡도: O(1)
+    public void reorderList(ListNode head) {
+
+        // Slow, Fast Pointer
+        // 1 - 2 - 3 - 4 - 5
+        ListNode slow = head;
+        ListNode fast = head;
+
+        // fast가 끝까지 닿을 때 중점 찾기
+        // slow = 3
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+
+
+        // 중간 이후부터 끝까지 (second half of list) => reverse order
+        // 3 이후부터 -> 4 - 5
+        // reverse - 5 - 4
+        ListNode backHead = null; // second half of list's new head
+        ListNode backSide = slow.next;
+        slow.next = null;
+
+        while (backSide != null) {
+            ListNode temp = backSide.next;
+            backSide.next = backHead;
+            backHead = backSide;
+            backSide = temp;
+        }
+
+        // Merge first and second half
+
+        // 1 - 2 - 3    
+        ListNode firstHalf = head;
+        // 5 - 4
+        ListNode secondHalf = backHead;
+
+        // 1 - 5 - 2 - 4 - 3
+        while (secondHalf != null) {
+            ListNode temp = firstHalf.next;
+            firstHalf.next = secondHalf;
+            firstHalf = secondHalf;
+            secondHalf = temp;
+        }
+    }
+}
+
+