From 071b8b5fd0ce8a33f7299ca5a48d59827a352499 Mon Sep 17 00:00:00 2001
From: Bofei Zhu <zhu.bofei@gmail.com>
Date: Sat, 11 Aug 2018 19:58:52 -0400
Subject: [PATCH] 024-swap-nodes-in-pairs

---
 .../Contents.swift                            | 62 +++++++++++++++++++
 .../contents.xcplayground                     |  4 ++
 README.md                                     |  1 +
 3 files changed, 67 insertions(+)
 create mode 100644 024-swap-nodes-in-pairs.playground/Contents.swift
 create mode 100644 024-swap-nodes-in-pairs.playground/contents.xcplayground

diff --git a/024-swap-nodes-in-pairs.playground/Contents.swift b/024-swap-nodes-in-pairs.playground/Contents.swift
new file mode 100644
index 0000000..73225b1
--- /dev/null
+++ b/024-swap-nodes-in-pairs.playground/Contents.swift
@@ -0,0 +1,62 @@
+/// 24. Swap Nodes in Pairs
+/// Given a linked list, swap every two adjacent nodes and return its head.
+///
+/// Example:
+/// Given 1->2->3->4, you should return the list as 2->1->4->3.
+/// - Note:
+///     Your algorithm should use only constant extra space.
+///     You may not modify the values in the list's nodes, only nodes itself may be changed.
+
+import XCTest
+
+func swapPairs(_ head: ListNode?) -> ListNode? {
+    let dummyHead = ListNode(0)
+    dummyHead.next = head
+    var current: ListNode? = dummyHead
+    while let left = current?.next,
+          let right = left.next {
+        left.next = right.next
+        right.next = left
+        current?.next = right
+        current = left
+    }
+    return dummyHead.next
+}
+
+final class ListNode {
+    var val: Int
+    var next: ListNode?
+    init(_ val: Int) {
+        self.val = val
+    }
+    
+    init(array: [Int]) {
+        guard !array.isEmpty else { fatalError() }
+        val = array[0]
+        var previous = self
+        for i in 1..<array.count {
+            let new = ListNode(array[i])
+            previous.next = new
+            previous = new
+        }
+    }
+    
+    func toArray() -> [Int] {
+        var current: ListNode? = self
+        var array: [Int] = []
+        while current != nil {
+            array.append(current!.val)
+            current = current?.next
+        }
+        return array
+    }
+}
+
+class Tests: XCTestCase {
+    func testListNodeInitCorretly() {
+        let head = ListNode(array: [1, 2, 3, 4])
+        XCTAssertEqual(swapPairs(head)?.toArray(), [2, 1, 4, 3])
+    }
+}
+
+Tests.defaultTestSuite.run()
diff --git a/024-swap-nodes-in-pairs.playground/contents.xcplayground b/024-swap-nodes-in-pairs.playground/contents.xcplayground
new file mode 100644
index 0000000..a93d484
--- /dev/null
+++ b/024-swap-nodes-in-pairs.playground/contents.xcplayground
@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' executeOnSourceChanges='false'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>
\ No newline at end of file
diff --git a/README.md b/README.md
index e154e05..ffb2337 100644
--- a/README.md
+++ b/README.md
@@ -39,3 +39,4 @@
 21 | [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/021-merge-two-sorted-lists.playground/Contents.swift)
 22 | [Generate Parentheses](https://leetcode.com/problems/generate-parentheses/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/022-generate-parentheses.playground/Contents.swift)
 23 | [Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/023-merge-k-sorted-lists.playground/Contents.swift)
+24 | [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/023-swap-nodes-in-pairs.playground/Contents.swift)