018-4sum

015-3sum.playground/Contents.swift

@@ -48,21 +48,6 @@ func threeSum(_ nums: [Int]) -> [[Int]] {
     return Array(set).map { $0.toArray() }
 }
 
-class Tests: XCTestCase {
-    func testExample() {
-        let nums = [-1, 0, 1, 2, -1, -4]
-        let expected = [
-            [-1, 0, 1],
-            [-1, -1, 2],
-        ]
-        let solutions = threeSum(nums)
-        XCTAssertEqual(solutions.count, expected.count)
-        XCTAssertEqual(Set(solutions), Set(expected))
-    }
-}
-
-Tests.defaultTestSuite.run()
-
 func threeSum4_2(_ nums: [Int]) -> [[Int]] {
     let nums = nums.sorted()
     var set: Set<[Int]> = []
@@ -84,3 +69,18 @@ func threeSum4_2(_ nums: [Int]) -> [[Int]] {
     }
     return Array(set)
 }
+
+class Tests: XCTestCase {
+    func testExample() {
+        let nums = [-1, 0, 1, 2, -1, -4]
+        let expected = [
+            [-1, 0, 1],
+            [-1, -1, 2],
+        ]
+        let solutions = threeSum(nums)
+        XCTAssertEqual(solutions.count, expected.count)
+        XCTAssertEqual(Set(solutions), Set(expected))
+    }
+}
+
+Tests.defaultTestSuite.run()

018-4sum.playground/Contents.swift

@@ -0,0 +1,94 @@
+/// 18. 4Sum
+/// Given an array nums of n integers and an integer target, are there elements a, b, c, and d in
+/// nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the
+/// sum of target.
+///
+/// - Note: The solution set must not contain duplicate quadruplets.
+
+import XCTest
+
+// Approach: Two Pointers
+func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
+    // [Int] is not Hashable in LeetCode runtime (Requires Swift 4.2).
+    // This is a workaround. The Swift 4.2 solution is attached below.
+    guard nums.count > 3 else { return [] }
+
+    struct Four: Hashable {
+        let a: Int
+        let b: Int
+        let c: Int
+        let d: Int
+
+        init(_ a: Int, _ b: Int, _ c: Int, _ d: Int) {
+            self.a = a
+            self.b = b
+            self.c = c
+            self.d = d
+        }
+
+        func toArray() -> [Int] {
+            return [a, b, c, d]
+        }
+    }
+    let nums = nums.sorted()
+    var set: Set<Four> = []
+    for i in 0...nums.count - 4 {
+        for j in i+1...nums.count - 3 {
+            var head = j + 1
+            var tail = nums.count - 1
+            while head < tail {
+                let sum = nums[i] + nums[j] + nums[head] + nums[tail]
+                if sum == target {
+                    set.insert(Four(nums[i], nums[j], nums[head], nums[tail]))
+                    head += 1
+                    tail -= 1
+                } else if sum > target {
+                    tail -= 1
+                } else {
+                    head += 1
+                }
+            }
+        }
+    }
+    return Array(set).map { $0.toArray() }
+}
+
+func fourSum4_2(_ nums: [Int], _ target: Int) -> [[Int]] {
+    let nums = nums.sorted()
+    var set: Set<[Int]> = []
+    for i in 0...nums.count - 3 {
+        for j in i+1...nums.count - 3 {
+            var head = j + 1
+            var tail = nums.count - 1
+            while head < tail {
+                let sum = nums[i] + nums[j] + nums[head] + nums[tail]
+                if sum == target {
+                    set.insert([nums[i], nums[j], nums[head], nums[tail]])
+                    head += 1
+                    tail -= 1
+                } else if sum > target {
+                    tail -= 1
+                } else {
+                    head += 1
+                }
+            }
+        }
+    }
+    return Array(set)
+}
+
+class Tests: XCTestCase {
+    func testExample() {
+        let nums = [1, 0, -1, 0, -2, 2]
+        let expected = [
+            [-1,  0, 0, 1],
+            [-2, -1, 1, 2],
+            [-2,  0, 0, 2],
+        ]
+        let solutions = fourSum(nums, 0)
+        XCTAssertEqual(solutions.count, expected.count)
+        XCTAssertEqual(Set(solutions), Set(expected))
+    }
+}
+
+Tests.defaultTestSuite.run()

018-4sum.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' executeOnSourceChanges='false'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

README.md

@@ -30,3 +30,4 @@
 15 | [Three Sum](https://leetcode.com/problems/3sum/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/015-3sum.playground/Contents.swift)
 16 | [Three Sum Closest](https://leetcode.com/problems/3sum-closest/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/016-3sum-closest.playground/Contents.swift)
 17 | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/017-letter-combinations-of-a-phone-number.playground/Contents.swift)
+18 | [Four Sum](https://leetcode.com/problems/4sum/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/018-4sum.playground/Contents.swift)