reorder-list

Author: chjung99

reorder-list/chjung99.java

@@ -0,0 +1,57 @@
+/**
+ * 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 {
+    public void reorderList(ListNode head) {
+        Map<Integer, ListNode> order = new HashMap<>();
+        ListNode start = head;
+        int n = 0;
+
+        while (start != null) {
+            order.put(n, start);
+            start = start.next;
+            n++;
+        }
+
+        ListNode cur = head;
+        List<Integer> newOrder = generateOrder(n);
+
+        for (int i = 0; i < n; i++) {
+            int idx = newOrder.get(i);
+            cur = order.get(idx);
+            if (i + 1 < n) {
+                cur.next = order.get(newOrder.get(i+1));
+            } else {
+                cur.next = null;
+            }
+            cur = cur.next;
+        }
+    }
+    public List<Integer> generateOrder(int n) {
+        List<Integer> reorder = new ArrayList<>();
+        int[] asc = new int[n];
+        int[] desc = new int[n];
+
+        for (int i = 0; i < n; i++) {
+            asc[i] = i;
+            desc[i] = n - 1 - i;
+        }
+
+        for (int i = 0; i< n; i++) {
+            if (i % 2 == 0) {
+                reorder.add(asc[i/2]);
+            } else {
+                reorder.add(desc[i/2]);
+            }
+        }
+        return reorder;
+    }
+}
+