[Hyun] Week 8 Solution Explanation

combination-sum/WhiteHyun.swift

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+//  
+//  39. Combination Sum
+//  https://leetcode.com/problems/combination-sum/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/27.
+//  
+
+class Solution {
+  func combinationSum(_ candidates: [Int], _ target: Int) -> [[Int]] {
+    let sortedCandidates = candidates.sorted()
+    var answer: [[Int]] = []
+
+    func backtrack(_ sum: Int, _ start: Int, _ current: [Int]) {
+      if sum == target {
+        answer.append(current)
+        return
+      }
+
+      if sum > target {
+        return
+      }
+
+      for i in start ..< sortedCandidates.count {
+        let newSum = sum + sortedCandidates[i]
+        if newSum > target {
+          break
+        }
+        backtrack(newSum, i, current + [sortedCandidates[i]])
+      }
+    }
+
+    backtrack(0, 0, [])
+    return answer
+  }
+}

construct-binary-tree-from-preorder-and-inorder-traversal/WhiteHyun.swift

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+//  
+//  105. Construct Binary Tree from Preorder and Inorder Traversal
+//  https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/27.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+class Solution {
+  func buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
+    let indices: [Int: Int] = Dictionary(uniqueKeysWithValues: inorder.enumerated().map { ($0.element, $0.offset) })
+
+    var preorderIndex = 0
+
+    func build(_ left: Int, _ right: Int) -> TreeNode? {
+      if left > right { return nil }
+
+      let rootValue = preorder[preorderIndex]
+      preorderIndex += 1
+      let root = TreeNode(rootValue)
+
+      let mid = indices[rootValue]!
+
+      root.left = build(left, mid - 1)
+      root.right = build(mid + 1, right)
+
+      return root
+    }
+
+    return build(0, inorder.count - 1)
+  }
+}

implement-trie-prefix-tree/WhiteHyun.swift

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+//  
+//  208. Implement Trie (Prefix Tree)
+//  https://leetcode.com/problems/implement-trie-prefix-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/28.
+//  
+
+// MARK: - Node
+
+private class Node {
+  var children: [Character: Node]
+  let value: Character
+  var isEndOfWord: Bool
+
+  init(children: [Character: Node] = [:], value: Character, isEndOfWord: Bool = false) {
+    self.children = children
+    self.value = value
+    self.isEndOfWord = isEndOfWord
+  }
+}
+
+// MARK: - Trie
+
+class Trie {
+  private var root: Node
+
+  init() {
+    root = .init(value: "$")
+  }
+
+  func insert(_ word: String) {
+    var node = root
+    for character in word {
+      if node.children[character] == nil {
+        node.children[character] = Node(value: character)
+      }
+      node = node.children[character]!
+    }
+    node.isEndOfWord = true
+  }
+
+  func search(_ word: String) -> Bool {
+    var node = root
+    for character in word {
+      if node.children[character] == nil {
+        return false
+      }
+      node = node.children[character]!
+    }
+    return node.isEndOfWord
+  }
+
+  func startsWith(_ prefix: String) -> Bool {
+    var node = root
+    for character in prefix {
+      if node.children[character] == nil {
+        return false
+      }
+      node = node.children[character]!
+    }
+    return true
+  }
+}

kth-smallest-element-in-a-bst/WhiteHyun.swift

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+//  
+//  230. Kth Smallest Element in a BST
+//  https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/27.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+class Solution {
+  private var triedCount: Int = 0
+  func kthSmallest(_ node: TreeNode?, _ k: Int) -> Int {
+    guard let node else { return -1 }
+    let number = kthSmallest(node.left, k)
+    if number != -1 { return number }
+    if triedCount + 1 == k { return node.val }
+    triedCount += 1
+    return kthSmallest(node.right, k)
+  }
+}

word-search/WhiteHyun.swift

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+//  
+//  79. Word Search
+//  https://leetcode.com/problems/word-search/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/28.
+//  
+
+class Solution {
+  func exist(_ board: [[Character]], _ word: String) -> Bool {
+    let rows = board.count
+    let cols = board[0].count
+    let word = Array(word)
+    var visited = Array(repeating: Array(repeating: false, count: cols), count: rows)
+
+    for row in 0 ..< rows {
+      for col in 0 ..< cols {
+        if board[row][col] == word[0], dfs(board, word, row, col, 0, &visited) {
+          return true
+        }
+      }
+    }
+
+    return false
+  }
+
+  private func dfs(_ board: [[Character]], _ word: [Character], _ row: Int, _ col: Int, _ index: Int, _ visited: inout [[Bool]]) -> Bool {
+    if index == word.count {
+      return true
+    }
+
+    if row < 0 || row >= board.count || col < 0 || col >= board[0].count || visited[row][col] || board[row][col] != word[index] {
+      return false
+    }
+
+    visited[row][col] = true
+
+    let directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]
+
+    for (dx, dy) in directions {
+      if dfs(board, word, row + dx, col + dy, index + 1, &visited) {
+        return true
+      }
+    }
+
+    visited[row][col] = false
+    return false
+  }
+}