[Hyun] Week 5 Solution Explanation

3sum/WhiteHyun.swift

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+//  
+//  15. 3Sum
+//  https://leetcode.com/problems/3sum/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/04.
+//  
+
+class Solution {
+  func threeSum(_ nums: [Int]) -> [[Int]] {
+    var result: [[Int]] = []
+    let sorted = nums.sorted()
+
+    for (index, element) in sorted.enumerated() where index <= 0 || element != sorted[index - 1] {
+      var left = index + 1
+      var right = sorted.count - 1
+
+      while left < right {
+        let threeSum = element + sorted[left] + sorted[right]
+        if threeSum > 0 {
+          right -= 1
+          continue
+        }
+        if threeSum < 0 {
+          left += 1
+          continue
+        }
+
+        result.append([element, sorted[left], sorted[right]])
+        left += 1
+
+        while sorted[left] == sorted[left - 1] && left < right {
+          left += 1
+        }
+      }
+    }
+
+
+    return result
+  }
+}

longest-consecutive-sequence/WhiteHyun.swift

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+//  
+//  128. Longest Consecutive Sequence
+//  https://leetcode.com/problems/longest-consecutive-sequence/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/01.
+//  
+
+final class Solution {
+  func longestConsecutive(_ nums: [Int]) -> Int {
+    let set = Set(nums)
+    var best = 0
+    for number in set where !set.contains(number - 1) {
+      var next = number + 1
+      while set.contains(next) {
+        next += 1
+      }
+      if best < next - number {
+        best = next - number
+      }
+    }
+
+    return best
+  }
+}

product-of-array-except-self/WhiteHyun.swift

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+//  
+//  238. Product of Array Except Self
+//  https://leetcode.com/problems/product-of-array-except-self/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/01.
+//  
+
+final class Solution {
+  func productExceptSelf(_ nums: [Int]) -> [Int] {
+    var answer: [Int] = .init(repeating: 1, count: nums.count)
+
+    var left_product = 1
+    for i in nums.indices {
+      answer[i] = left_product
+      left_product *= nums[i]
+    }
+
+    var right_product = 1
+    for i in nums.indices.reversed() {
+      answer[i] *= right_product
+      right_product *= nums[i]
+    }
+
+    return answer
+  }
+}

top-k-frequent-elements/WhiteHyun.swift

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+//  
+//  347. Top K Frequent Elements
+//  https://leetcode.com/problems/top-k-frequent-elements/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/01.
+//  
+
+final class Solution {
+  func topKFrequent(_ nums: [Int], _ k: Int) -> [Int] {
+    // Python의 Counter 모듈처럼 만드는 것과 동일
+    var counter: [Int: Int] = [:]
+    for number in nums {
+      counter[number, default: 0] += 1
+    }
+
+    return counter
+      .sorted { lhs, rhs in
+        lhs.value > rhs.value                             // 빈도 수를 기준으로 내림차순 정렬
+      }
+      .prefix(k)                                          // 앞에서부터 k만큼 Subarray로 가져옴
+      .map(\.key)                                         // 제일 빈도가 많았던 것의 숫자를 가져옴 (Dictionary의 Key값)
+  }
+}