[Hyun] Week 4 Solution Explanation

counting-bits/WhiteHyun.swift

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+//  
+//  338. Counting Bits
+//  https://leetcode.com/problems/counting-bits/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+
+  // MARK: - Time Complexity: O(n), Space Complexity: O(n)
+
+  func countBits(_ n: Int) -> [Int] {
+    var array: [Int] = .init(repeating: 0, count: n + 1)
+    for i in stride(from: 1, through: n, by: 1) {
+      array[i] = array[i >> 1] + (i & 1)
+    }
+    return array
+  }
+
+  // MARK: - nonzeroBitCount
+
+  func countBits2(_ n: Int) -> [Int] {
+    return (0...n).map(\.nonzeroBitCount)
+  }
+
+}

group-anagrams/WhiteHyun.swift

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+//  
+//  49. Group Anagrams
+//  https://leetcode.com/problems/group-anagrams/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+  func groupAnagrams(_ strs: [String]) -> [[String]] {
+    var dictionary: [String: [String]] = [:]
+    for str in strs {
+      dictionary[String(str.sorted()), default: []].append(str)
+    }
+
+    return Array(dictionary.values)
+  }
+}

missing-number/WhiteHyun.swift

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+//  
+//  268. Missing Number
+//  https://leetcode.com/problems/missing-number/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+  func missingNumber(_ nums: [Int]) -> Int {
+    (0 ... nums.count).reduce(0, +) - nums.reduce(0, +)
+  }
+
+  func missingNumber2(_ nums: [Int]) -> Int {
+    nums.count * (nums.count + 1) / 2 - nums.reduce(0, +)
+  }
+
+  func missingNumber3(_ nums: [Int]) -> Int {
+    Set(0...nums.count).subtracting(Set(nums)).first!
+  }
+
+  func missingNumber4(_ nums: [Int]) -> Int {
+    var answer = 0
+    for i in 0 ..< nums.count {
+      answer = answer ^ i ^ nums[i]
+    }
+
+    return answer ^ nums.count
+  }
+}

number-of-1-bits/WhiteHyun.swift

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+//  
+//  191. Number of 1 Bits
+//  https://leetcode.com/problems/number-of-1-bits/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+  func hammingWeight(_ n: Int) -> Int {
+    n.nonzeroBitCount
+  }
+
+  func hammingWeight2(_ n: Int) -> Int {
+    var number = n
+    var answer = 0
+    while number != 0 {
+      answer += number & 1
+      number >>= 1
+    }
+
+    return answer
+  }
+}

reverse-bits/WhiteHyun.swift

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+//  
+//  190. Reverse Bits
+//  https://leetcode.com/problems/reverse-bits/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+
+  // MARK: - Runtime: 5ms / Memory 16.12 MB
+
+  func reverseBits(_ n: Int) -> Int {
+    let reversedStringBits = String(String(n, radix: 2).reversed())
+    return Int(reversedStringBits + String(repeating: "0", count: 32 - reversedStringBits.count), radix: 2)!
+  }
+
+  // MARK: - Runtime: 5ms / Memory 15.72 MB
+
+  func reverseBits2(_ n: Int) -> Int {
+    var answer = 0
+
+    for index in 0 ..< 32 {
+      answer += ((n >> (32 - index - 1)) & 1) << index
+    }
+    return answer
+  }
+}