update q206

src/链表操作/q206_反转链表/f1/Solution.java

@@ -1,28 +1,19 @@
 package 链表操作.q206_反转链表.f1;
 
-import java.util.ArrayList;
-import java.util.List;
-
 /**
- * 暴力法舍弃空间 o(n)
+ * 遍历直接反向修改next指针 o(n)
  */
 class Solution {
+
     public ListNode reverseList(ListNode head) {
-        if (head == null || head.next == null) {
-            return head;
-        }
-        List<Integer> list = new ArrayList<>();
+        ListNode pre  = null;
         ListNode temp = head;
         while (temp != null) {
-            list.add(temp.val);
-            temp = temp.next;
-        }
-        ListNode rs = new ListNode(list.get(list.size() - 1));
-        ListNode t1 = rs;
-        for (int i = list.size() - 2; i >= 0; i--) {
-            t1.next = new ListNode(list.get(i));
-            t1 = t1.next;
+            ListNode t = temp.next;
+            temp.next = pre;
+            pre = temp;
+            temp = t;
         }
-        return rs;
+        return pre;
     }
 }

src/链表操作/q206_反转链表/f2/Solution.java

@@ -1,18 +1,17 @@
 package 链表操作.q206_反转链表.f2;
 
 /**
- * 遍历直接反向修改next指针 o(n)
+ * 递归法 o(n)
  */
 class Solution {
+
     public ListNode reverseList(ListNode head) {
-        ListNode pre  = null;
-        ListNode temp = head;
-        while (temp != null) {
-            ListNode t = temp.next;
-            temp.next = pre;
-            pre = temp;
-            temp = t;
+        if (head == null || head.next == null) {
+            return head;
         }
-        return pre;
+        ListNode p = reverseList(head.next);
+        head.next.next = head;
+        head.next = null;
+        return p;
     }
 }