SwiftCodingCheatSheet

Array [Int](1...3) // [1, 2, 3] [1, 2] + [3] // [1, 2, 3] (arr[i], arr[j]) = (arr[j], arr[i]) // arr.swapAt(i,j) Array fast remove arr.swapAt(i, arr.count-1) arr.removeLast() 2D Array var arr3x3 = Array(repeating: [Int](0..<3), count:3) var arrRow1 = arr3x3[1] arrRow1[1] = 4 arr3x3[1] = arrRow1 // must set back // or arr3x3[1][1] = 4 Array as Queue arr.first, arr.last // Optional if !arr.isEmpty { let f = arr.removeFirst() //let e = arr.removeLast() } arr.remove(at: i) arr.insert(e, at: 0) arr.append(e) Sub Array // arr: [1, 2, 3, 4] var rest: ArraySlice = arr.dropFirst(2) // [3, 4] rest = arr[2..<array.count] //arr[2...array.count-1] let restArr = Array(rest) // first k subarray: Array(arr[0..<k]) String str.count str.append("a") // str += "b" // get character at index // str[str.index(str.startIndex, offsetBy:index)] let arr = Array(str); arr[index] String <-> Int Int(str) String(i) "\(i)" String <-> Array Array(str) let a:[Character] = ["a", "b"] String(a) let b = ["a", "b"] b.joined() b.joined(separator: "") Character for (i, c) in str.enumerated() where c == " " { let a = Int(c.asciiValue! /*UInt8?*/) } for c in str.unicodeScalars { let _ = Int(c.value /*UInt32*/) } Reorder var arr = [1, 3, 2] arr.sort()// mutating func sort() arr.sort(by: >) // [3, 2, 1] let sorted = arr.sorted() // func sorted() -> [Element] reverse() / reversed() / shuffle() / shuffled()

Dictionary var dic = [String: Int]() if let _ = dic[key] { dic[key] = nil // dic.removeValue(forKey:key) } dic[key, default:0] += 1 // sort keys by value let sortedKeys = dic.keys.sorted { return dic[$0]! > dic[$1]! } // To Array Array(dic.values) for value in dic.values {} Set var s = Set(arr) s.isEmpty s.contains(e) s.insert(e) s.remove(e) union、intersection、subtracting、symmetricDifference Random Int.random(in: 0..<6) arr.randomElement() Numbers max(4, 2, 1, 3, 5) min(1.0, 2.1) Int.min Int.max Enumeration for i in 1...10 {} for i in (0..<10).reversed() {} for i in stride(from:0, to:10, by:2){} // 0 2 4 6 8 for i in stride(from: 2, through: 0, by: -1) {} // 2 1 0 for (i, element) in arr.enumerated() {} for (i, element) in arr.enumerated() where element > 3 {} for (value, char) in zip(65...66, ["A", "B", "C"]) {} // (65, A) (66, B) let sum = nums.reduce(0, { x, y in x ^ y }) // nums.reduce(0, ^) Templates func dfs(_ result: inout [[Int]], _ path: inout [Int], _ visited: inout [Bool], _ arr: [Int], _ index: Int) { // if end condition { result.append(path); return } for i in 0..<arr.count { // if should filter out { continue } visited[i] = true path.append(nums[i]) dfs(&result, &path, &visited, arr, i+1) path.removeLast() visited[i] = false } } // Trie Node class TrieNode { var children = [Character: TrieNode]() var isEnd = false } // Hashable extension TreeNode: Hashable { public static func ==(l: TreeNode, r: TreeNode) -> Bool { return l === r } public func hash(into hasher: inout Hasher) { hasher.combine(ObjectIdentifier(self).hashValue) } } var cache = [TreeNode: Int]() // 151. Reverse Words in a String s.split(separator: " ").reversed().joined(separator: " ") // monotonic stack (next greater element) var maxQueue = [Int](), result = [Int](repeating:-1, count:nums.count) for (i, num) in nums.enumerated() { while (!maxQueue.isEmpty && num > nums[maxQueue.last!]) { result[maxQueue.last!] = num maxQueue.removeLast() } maxQueue.append(i) }