countingSort in swift

swift/14_sorts/CountingSort.swift

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+//
+//  CountingSort.swift
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/18.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+import Foundation
+
+// 假设数组中储存的都是非负整数
+public func countingSort(_ a: inout [Int]) {
+    if a.count <= 1 { return }
+
+    var counts = Array(repeating: 0, count: a.max()! + 1)
+    for num in a {
+        counts[num] += 1
+    }
+    for i in 1..<counts.count {
+        counts[i] = counts[i-1] + counts[i]
+    }
+
+    var aSorted = Array(repeating: 0, count: a.count)
+    for num in a.reversed() {
+        let index = counts[num] - 1
+        aSorted[index] = num
+        counts[num] -= 1
+    }
+
+    for (i, num) in aSorted.enumerated() {
+        a[i] = num
+    }
+}