Reverse list

Author: caipengbo

src/list/manip/P25ReverseNodesInKGroup.java

@@ -0,0 +1,39 @@
+package list.manip;
+
+import util.ListNode;
+import util.ListUtil;
+
+/**
+ * Title: 25. K个一组反转链表 Desc: 给你一个链表，每 k 个节点一组进行翻转，请你返回翻转后的链表。
+ * k 是一个正整数，它的值小于或等于链表的长度。 如果节点总数不是 k 的整数倍，那么请将最后剩余的节点保持原有顺序。 Created by Myth-PC
+ * on 08/02/2020 in VSCode
+ */
+public class P25ReverseNodesInKGroup {
+    public ListNode reverseKGroup(ListNode head, int k) {
+        ListNode p = head, q = head;
+        // 找到 第 k-1个结点p
+        for (int i = 0; i < k; i++) {
+            if (p == null) return head;
+            q = p;
+            p = p.next;
+        }
+        q.next = null;
+        ListNode newHead = reverse(head);
+        head.next = reverseKGroup(p, k);
+        return newHead;
+    }
+    private ListNode reverse(ListNode head) {
+        if (head == null || head.next == null) return head;
+        ListNode ret = reverse(head.next);
+        head.next.next = head;
+        head.next = null;
+        return ret;
+    }
+    public static void main(String[] args) {
+        ListNode list = ListUtil.stringToListNode("[1,2,3,4,5]");
+        P25ReverseNodesInKGroup p25 = new P25ReverseNodesInKGroup();
+        ListNode head = p25.reverseKGroup(list, 2);
+        System.out.println(list.val);
+        ListUtil.prettyPrintLinkedList(head);
+    }
+}

src/list/manip/P92ReverseLinkedList2.java

@@ -0,0 +1,57 @@
+package list.manip;
+
+import util.ListNode;
+import util.ListUtil;
+
+/**
+ * Title: 92. 反转链表2
+ * Desc: 反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。
+ * Created by Myth-PC on 08/02/2020 in VSCode
+ */
+public class P92ReverseLinkedList2 {
+    public ListNode reverseBetween(ListNode head, int m, int n) {
+        ListNode oldLast = head, before = null, cur, after;
+        
+        for (int i = 1; i < m-1; i++) {
+            oldLast = oldLast.next;
+        }
+        ListNode reverseLast = (m == 1 ? head : oldLast.next);
+        cur = reverseLast;
+        for (int i = m; i <= n; i++) {
+            after = cur.next;
+            cur.next = before;
+            before = cur;
+            cur = after;
+        }
+        reverseLast.next = cur;
+        if (m == 1) return before;
+        oldLast.next = before;
+        return head;
+    }
+    // 设置DummyNode
+    public ListNode reverseBetween2(ListNode head, int m, int n) {
+        ListNode dummy = new ListNode(-1);
+        dummy.next = head;
+        ListNode oldLast = dummy, before = null, cur, after;
+        
+        for (int i = 1; i < m; i++) {
+            oldLast = oldLast.next;
+        }
+        ListNode reverseLast = oldLast.next;
+        cur = reverseLast;
+        for (int i = m; i <= n; i++) {
+            after = cur.next;
+            cur.next = before;
+            before = cur;
+            cur = after;
+        }
+        reverseLast.next = cur;
+        oldLast.next = before;
+        return dummy.next;
+    }
+    public static void main(String[] args) {
+        P92ReverseLinkedList2 p92 = new P92ReverseLinkedList2();
+        ListNode head = ListUtil.stringToListNode("[1,2,3,4,5]");
+        ListUtil.prettyPrintLinkedList(p92.reverseBetween2(head, 1, 2));
+    }
+}

src/tree/README.md

@@ -2,4 +2,4 @@
 
 对于树，首先就要弄清前序、中序、后序
 
-使用的是递归，将相关操作放在前、中、后要认真思考
+基本上所有树的题目都是在遍历的基础上进行修改的