19-remove-nth-node-from-end-of-list

19-remove-nth-node-from-end-of-list.playground/Contents.swift

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+/// 19. Remove Nth Node From End of List
+/// Given a linked list, remove the n-th node from the end of list and return its head.
+///
+/// Example:
+/// Given linked list: 1->2->3->4->5, and n = 2.
+///
+/// After removing the second node from the end, the linked list becomes 1->2->3->5.
+/// - Note: Given n will always be valid.
+
+import XCTest
+
+/// - Warning: Swift judge for this problem isn't working. So the solution below hasn't been tested
+///     on LeetCode.com yet.
+func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
+    var nodes: [ListNode] = []
+    var next = head
+    while next != nil {
+        nodes.append(next!)
+        next = next?.next
+    }
+    let idx = nodes.count - n
+    guard idx > 0 else {
+        head?.next = nil
+        return nodes[1] // n will always be valid
+    }
+    nodes[idx - 1].next = nodes[idx].next
+    nodes[idx].next = nil
+    return head
+}
+
+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 testExample() {
+        let head = ListNode(array: [1, 2, 3, 4, 5])
+        let result = removeNthFromEnd(head, 2)
+        XCTAssertEqual(result?.toArray(), [1, 2, 3, 5])
+    }
+}
+
+Tests.defaultTestSuite.run()

19-remove-nth-node-from-end-of-list.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' executeOnSourceChanges='false'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>