我想了解3路基数Quicksort,我不明白为什么CUTOFF变量在那里?和插入方法?3路快速排序,问题
public class Quick3string {
private static final int CUTOFF = 15; // cutoff to insertion sort
// sort the array a[] of strings
public static void sort(String[] a) {
// StdRandom.shuffle(a);
sort(a, 0, a.length-1, 0);
assert isSorted(a);
}
// return the dth character of s, -1 if d = length of s
private static int charAt(String s, int d) {
assert d >= 0 && d <= s.length();
if (d == s.length()) return -1;
return s.charAt(d);
}
// 3-way string quicksort a[lo..hi] starting at dth character
private static void sort(String[] a, int lo, int hi, int d) {
// cutoff to insertion sort for small subarrays
if (hi <= lo + CUTOFF) {
insertion(a, lo, hi, d);
return;
}
int lt = lo, gt = hi;
int v = charAt(a[lo], d);
int i = lo + 1;
while (i <= gt) {
int t = charAt(a[i], d);
if (t < v) exch(a, lt++, i++);
else if (t > v) exch(a, i, gt--);
else i++;
}
// a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi].
sort(a, lo, lt-1, d);
if (v >= 0) sort(a, lt, gt, d+1);
sort(a, gt+1, hi, d);
}
// sort from a[lo] to a[hi], starting at the dth character
private static void insertion(String[] a, int lo, int hi, int d) {
for (int i = lo; i <= hi; i++)
for (int j = i; j > lo && less(a[j], a[j-1], d); j--)
exch(a, j, j-1);
}
// exchange a[i] and a[j]
private static void exch(String[] a, int i, int j) {
String temp = a[i];
a[i] = a[j];
a[j] = temp;
}
// is v less than w, starting at character d
private static boolean less(String v, String w, int d) {
assert v.substring(0, d).equals(w.substring(0, d));
return v.substring(d).compareTo(w.substring(d)) < 0;
}
// is the array sorted
private static boolean isSorted(String[] a) {
for (int i = 1; i < a.length; i++)
if (a[i].compareTo(a[i-1]) < 0) return false;
return true;
}
public static void main(String[] args) {
// read in the strings from standard input
String[] a = StdIn.readAll().split("\\s+");
int N = a.length;
// sort the strings
sort(a);
// print the results
for (int i = 0; i < N; i++)
StdOut.println(a[i]);
}
}
从http://www.cs.princeton.edu/algs4/51radix/Quick3string.java.html
@peiska:不是答案(因此评论)......八十年代的“优化”有多可爱;)在多核CPU的这个时代,真正的优化是通过并行获得的。我写了自己正确的Java多线程quicksort(现在在数百个系统上生产),并* *是*优化:)我在这里谈了一下:http://stackoverflow.com/questions/2210185看到你'重新询问quicksort,我认为值得一提的是,现在最快的quicksort(对于实际数据量)绝对是多线程的。 – SyntaxT3rr0r 2010-06-10 18:20:03
请注意,CUTOFF的值很可能会从帽子中取出。 – 2010-06-10 18:34:20
@ThorbjørnRavnAndersenDon Knuth的帽子,其实是带着证明。 – EJP 2013-10-17 22:35:57