016-3sum-closest

016-3sum-closest.playground/Contents.swift

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+/// 16. 3Sum Closest
+/// Given an array nums of n integers and an integer target, find three integers in nums such that
+/// the sum is closest to target. Return the sum of the three integers. You may assume that each
+/// input would have exactly one solution.
+
+import XCTest
+
+// Approach: Two Pointers
+func threeSumClosest(_ nums: [Int], _ target: Int) -> Int {
+    guard nums.count > 2 else { fatalError() }
+    let nums = nums.sorted()
+    var closestSum = nums[0] + nums[1] + nums[2]
+    for i in 0...nums.count - 3 {
+        var head = i + 1
+        var tail = nums.count - 1
+        while head < tail {
+            let sum = nums[i] + nums[head] + nums[tail]
+
+            // update pointers
+            if sum == target {
+                return target
+            } else if sum > target {
+                tail -= 1
+            } else {
+                head += 1
+            }
+
+            // update closest sum
+            closestSum = abs(sum - target) < abs(closestSum - target) ? sum : closestSum
+        }
+    }
+    return closestSum
+}
+
+class Tests: XCTestCase {
+    func testExample() {
+        let nums = [-1, 2, 1, -4]
+        XCTAssertEqual(threeSumClosest(nums, 1), 2)
+    }
+}
+
+Tests.defaultTestSuite.run()

016-3sum-closest.playground/contents.xcplayground

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