[Hyun] Week 2 Solution Explanation

invert-binary-tree/WhiteHyun.swift

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+//
+//  226. Invert Binary Tree.swift
+//  https://leetcode.com/problems/invert-binary-tree/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode226 {
+  func invertTree(_ node: TreeNode?) -> TreeNode? {
+    guard let node else { return nil }
+
+    let left = node.left
+    let right = node.right
+
+    node.left = invertTree(right)
+    node.right = invertTree(left)
+
+    return node
+  }
+}

linked-list-cycle/WhiteHyun.swift

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+//
+//  141. Linked List Cycle.swift
+//  https://leetcode.com/problems/linked-list-cycle/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode141 {
+  func hasCycle(_ head: ListNode?) -> Bool {
+    guard head != nil, head?.next != nil
+    else {
+      return false
+    }
+
+    var tortoise = head
+    var hare = head?.next
+
+    while hare != nil, hare?.next != nil {
+      if tortoise === hare { return true }
+      tortoise = tortoise?.next
+      hare = hare?.next?.next
+    }
+
+    return false
+  }
+}

merge-two-sorted-lists/WhiteHyun.swift

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+//
+//  21. Merge Two Sorted Lists.swift
+//  https://leetcode.com/problems/merge-two-sorted-lists/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode21 {
+  func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
+    let dummy: ListNode? = .init(0)
+    var currentNode: ListNode? = dummy
+    var l1 = list1
+    var l2 = list2
+
+    while l1 != nil, l2 != nil {
+      if l1!.val < l2!.val {
+        currentNode?.next = l1
+        l1 = l1?.next
+      } else {
+        currentNode?.next = l2
+        l2 = l2?.next
+      }
+
+      currentNode = currentNode?.next
+    }
+
+    currentNode?.next = l1 ?? l2
+
+    return dummy?.next
+  }
+}

reverse-linked-list/WhiteHyun.swift

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+//
+//  206. Reverse Linked List.swift
+//  https://leetcode.com/problems/reverse-linked-list/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode206 {
+  func reverseList(_ node: ListNode?, _ prev: ListNode? = nil) -> ListNode? {
+    guard let node else { return prev }
+
+    let next = node.next
+    node.next = prev
+
+    return reverseList(next, node)
+  }
+}

valid-parentheses/WhiteHyun.swift

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+//
+//  20. Valid Parentheses.swift
+//  https://leetcode.com/problems/valid-parentheses/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode20 {
+  func isValid(_ s: String) -> Bool {
+    var stack: [Character] = []
+
+    for character in s {
+      if "([{".contains(character) {
+        stack.append(character)
+        continue
+      }
+
+      // `([{`가 아닌 닫힌 괄호가 들어왔는데 stack이 비어있으면 안 됨.
+      if stack.isEmpty { return false }
+
+      // 아스키 값의 차를 활용하여 1~2사이의 차이가 난다면 알맞은 쌍.
+      // 하지만, 각 괄호의 아스키 값이 3 이상 나는 경우는 서로 맞지 않음.
+      // Swift에서는 asciiValue 프로퍼티의 타입이 `UInt8` 이기 때문에 음수가 없어서 가능한 비교!
+      // `&-` 는 오버플로우 연산으로, 만약 오버플로우가 난다면 무시함.
+      if character.asciiValue! &- stack.last!.asciiValue! > 2 {
+        return false
+      }
+
+      stack.removeLast()
+    }
+
+    return stack.isEmpty
+  }
+}