LeetCode 24. Swap Nodes in Pairs

Author: raul-sauco

README.md

@@ -35,6 +35,7 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 | [21. Merge Two Sorted Lists][lc21]                                             | 🟢 Easy    | [![python](res/py.png)][lc21py]                                     |
 | [22. Generate Parentheses][lc22]                                               | 🟠 Medium  | [![python](res/py.png)][lc22py]                                     |
 | [23. Merge k Sorted Lists][lc23]                                               | 🔴 Hard    | [![python](res/py.png)][lc23py]                                     |
+| [24. Swap Nodes in Pairs][lc24]                                                | 🟠 Medium  | [![python](res/py.png)][lc24py]                                     |
 | [26. Remove Duplicates from Sorted Array][lc26]                                | 🟢 Easy    | [![python](res/py.png)][lc26py]                                     |
 | [27. Remove Element][lc27]                                                     | 🟢 Easy    | [![python](res/py.png)][lc27py]                                     |
 | [29. Divide two integers][lc29]                                                | 🟠 Medium  | [![python](res/py.png)][lc29py]                                     |
@@ -373,6 +374,8 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 [lc22py]: leetcode/generate-parentheses.py
 [lc23]: https://leetcode.com/problems/merge-k-sorted-lists/
 [lc23py]: leetcode/merge-k-sorted-lists.py
+[lc24]: https://leetcode.com/problems/swap-nodes-in-pairs/
+[lc24py]: leetcode/swap-nodes-in-pairs.py
 [lc26]: https://leetcode.com/problems/remove-duplicates-from-sorted-array/
 [lc26py]: leetcode/remove-duplicates-from-sorted-array.py
 [lc27]: https://leetcode.com/problems/remove-element/

leetcode/swap-nodes-in-pairs.py

@@ -0,0 +1,72 @@
+# 24. Swap Nodes in Pairs
+# 🟠 Medium
+#
+# https://leetcode.com/problems/swap-nodes-in-pairs/
+#
+# Tags: Linked List - Two Pointers
+
+import timeit
+from typing import Optional
+
+from data import LinkedList, ListNode
+
+
+# Keep a pointer to the node right before the current group, then swap
+# the pointers from the front node to the back node, and make the front
+# node point to the next node after the back one.
+#
+# Time complexity: O(n) - We iterate over the entire list one time.
+# Space complexity: O(1) - We only store pointers in memory.
+#
+# Runtime: 35 ms, faster than 89.78%
+# Memory Usage: 13.8 MB, less than 65.67%
+class Solution:
+    def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+        # Use four pointers.
+        dummy = ListNode(0, head)
+        prev, one, two, next = dummy, head, head.next, None
+        while one and two:
+            # Update next and prev pointers.
+            next, prev.next, prev = two.next, two, one
+            # Update internal group pointers.
+            one.next, two.next = two.next, one
+            # Shuffle working node pointers.
+            one = next
+            if one:
+                two = one.next
+        return dummy.next
+
+
+def test():
+    executors = [Solution]
+    tests = [
+        [[], []],
+        [[1], [1]],
+        [[1, 2], [2, 1]],
+        [[1, 2, 3], [2, 1, 3]],
+        [[1, 2, 3, 4], [2, 1, 4, 3]],
+        [[1, 2, 3, 4, 5], [2, 1, 4, 3, 5]],
+        [[1, 2, 3, 4, 5, 8], [2, 1, 4, 3, 8, 5]],
+    ]
+    for executor in executors:
+        start = timeit.default_timer()
+        for _ in range(1):
+            for col, t in enumerate(tests):
+                sol = executor()
+                head = LinkedList.fromList(t[0]).getHead()
+                result = LinkedList(sol.swapPairs(head)).toList()
+                exp = t[1]
+                assert result == exp, (
+                    f"\033[93m» {result} <> {exp}\033[91m for"
+                    + f" test {col} using \033[1m{executor.__name__}"
+                )
+        stop = timeit.default_timer()
+        used = str(round(stop - start, 5))
+        cols = "{0:20}{1:10}{2:10}"
+        res = cols.format(executor.__name__, used, "seconds")
+        print(f"\033[92m» {res}\033[0m")
+
+
+test()