递归学习

src/main/java/com/ximo/datastructuresinaction/leetcode/recursive/SwapNode.java

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+package com.ximo.datastructuresinaction.leetcode.recursive;
+
+import java.util.StringJoiner;
+
+/**
+ * 两两交换链表中的节点
+ *
+ * 给定一个链表，两两交换其中相邻的节点，并返回交换后的链表。
+ *
+ * 你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。
+ *
+ * 示例:
+ *
+ * 给定 1->2->3->4, 你应该返回 2->1->4->3.
+ *
+ * @author xikl
+ * @date 2019/7/3
+ */
+public class SwapNode {
+
+    private static class ListNode {
+        int value;
+        private ListNode tail;
+
+        public ListNode(int x) {
+            this.value = x;
+        }
+
+    }
+
+    public static ListNode swapPairs(ListNode head) {
+        if (head == null || head.tail == null) {
+            return head;
+        }
+
+        ListNode tail = head.tail;
+        head.tail = swapPairs(tail.tail);
+
+        tail.tail = head;
+
+        // 此时他就是头结点
+        return tail;
+    }
+
+    public static void main(String[] args) {
+        ListNode head = new ListNode(1);
+        head.tail = new ListNode(2);
+        head.tail.tail = new ListNode(3);
+        head.tail.tail.tail = new ListNode(4);
+        print(head);
+
+        ListNode result = swapPairs(head);
+        print(result);
+
+    }
+
+    private static void print(ListNode head) {
+        ListNode cur = head;
+        StringJoiner joiner = new StringJoiner(" -> ", "{", "}");
+        while (cur != null) {
+            joiner.add(String.valueOf(cur.value));
+            cur = cur.tail;
+        }
+        System.out.println(joiner.toString());
+    }
+
+
+}