25. K 个一组翻转链表

Author: houfachao

src/main/java/com/geekidentity/leetcode/n0025/ReverseKGroup.java

@@ -0,0 +1,45 @@
+package com.geekidentity.leetcode.n0025;
+
+public class ReverseKGroup {
+    public ListNode reverseKGroup(ListNode head, int k) {
+        if (head == null || head.next == null) return head;
+        ListNode dummy = new ListNode(0, head);
+        ListNode pre = dummy, end = dummy;
+        while (end.next != null) {
+            for (int i = 0; i < k && end != null; i ++) {
+                end = end.next;
+            }
+            if (end == null) break;;
+            ListNode start = pre.next;
+            ListNode next = end.next;
+            end.next = null;
+            ListNode newHead = reverse(start);
+            pre.next = newHead;
+            start.next = next;
+            pre = start;
+            end = pre;
+        }
+        return dummy.next;
+
+    }
+
+    public ListNode reverse(ListNode head) {
+        if (head == null || head.next == null) return head;
+        ListNode pre = null, cur = head, next = null;
+        while (cur != null) {
+            next = cur.next;
+            cur.next = pre;
+            pre = cur;
+            cur = next;
+        }
+        return pre;
+    }
+
+    static class ListNode {
+      int val;
+      ListNode next;
+      ListNode() {}
+      ListNode(int val) { this.val = val; }
+      ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+  }
+}