001
002
003
004
005
006
007
008
009
010
011
012
013
014
015
016
017
018
019
020
021
022
023
024
025
026
027
028
029
030
031
032
033
034
035
036
037
038
039
040
041
042
043
044
045
046
047
048
049
050
051
052
053
054
055
056
057
058
059
060
061
062
063
064
065
066
067
068
069
070
071
072
073
074
075
076
077
078
079
080
081
082
083
084
085
086
087
088
089
090
091
092
093
094
095
096
097
098
099
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
package algs23;
import stdlib.*;
/* ***********************************************************************
 *  Compilation:  javac Quick.java
 *  Execution:    java Quick < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   http://algs4.cs.princeton.edu/23quicksort/tiny.txt
 *                http://algs4.cs.princeton.edu/23quicksort/words3.txt
 *
 *  Sorts a sequence of strings from standard input using quicksort.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java Quick < tiny.txt
 *  A E E L M O P R S T X                 [ one string per line ]
 *
 *  % more words3.txt
 *  bed bug dad yes zoo ... all bad yet
 *
 *  % java Quick < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *************************************************************************/

public class Quick {

  // quicksort the array
  public static <T extends Comparable<? super T>> void sort(T[] a) {
    StdRandom.shuffle(a);
    sort(a, 0, a.length - 1);
  }

  // quicksort the subarray from a[lo] to a[hi]
  private static <T extends Comparable<? super T>> void sort(T[] a, int lo, int hi) {
    if (hi <= lo) return;
    int j = partition(a, lo, hi);
    sort(a, lo, j-1);
    sort(a, j+1, hi);
    assert isSorted(a, lo, hi);
  }

  // partition the subarray a[lo .. hi] by returning an index j
  // so that a[lo .. j-1] <= a[j] <= a[j+1 .. hi]
  private static <T extends Comparable<? super T>> int partition(T[] a, int lo, int hi) {
    int i = lo;
    int j = hi + 1;
    T v = a[lo];
    while (true) {

      // find item on lo to swap
      while (less(a[++i], v))
        if (i == hi) break;

      // find item on hi to swap
      while (less(v, a[--j]))
        if (j == lo) break;      // redundant since a[lo] acts as sentinel

      // check if pointers cross
      if (i >= j) break;

      exch(a, i, j);
    }

    // put v = a[j] into position
    exch(a, lo, j);

    // with a[lo .. j-1] <= a[j] <= a[j+1 .. hi]
    return j;
  }

  /* *********************************************************************
   *  Rearranges the elements in a so that a[k] is the kth smallest element,
   *  and a[0] through a[k-1] are less than or equal to a[k], and
   *  a[k+1] through a[n-1] are greater than or equal to a[k].
   ***********************************************************************/
  public static <T extends Comparable<? super T>> T select(T[] a, int k) {
    if (k < 0 || k >= a.length) {
      throw new Error("Selected element out of bounds");
    }
    StdRandom.shuffle(a);
    int lo = 0, hi = a.length - 1;
    while (hi > lo) {
      int i = partition(a, lo, hi);
      if      (i > k) hi = i - 1;
      else if (i < k) lo = i + 1;
      else return a[i];
    }
    return a[lo];
  }



  /* *********************************************************************
   *  Helper sorting functions
   ***********************************************************************/

  // is v < w ?
  private static <T extends Comparable<? super T>> boolean less(T v, T w) {
    if (COUNT_OPS) DoublingTest.incOps ();
    return (v.compareTo(w) < 0);
  }

  // exchange a[i] and a[j]
  private static <T> void exch(T[] a, int i, int j) {
    if (COUNT_OPS) DoublingTest.incOps ();
    T swap = a[i];
    a[i] = a[j];
    a[j] = swap;
  }


  /* *********************************************************************
   *  Check if array is sorted - useful for debugging
   ***********************************************************************/
  private static <T extends Comparable<? super T>> boolean isSorted(T[] a) {
    return isSorted(a, 0, a.length - 1);
  }

  private static <T extends Comparable<? super T>> boolean isSorted(T[] a, int lo, int hi) {
    for (int i = lo + 1; i <= hi; i++)
      if (less(a[i], a[i-1])) return false;
    return true;
  }


  // print array to standard output
  private static <T> void show(T[] a) {
    for (T element : a) {
      StdOut.println(element);
    }
  }

  // test code
  private static boolean COUNT_OPS = false;
  public static void main(String[] args) {
    //String[] cards = In.readStrings ("data/cards.txt");
    //StdRandom.shuffle (cards);

    //StdIn.fromFile ("data/tiny.txt");
    //StdIn.fromFile ("data/cards.txt");
    StdIn.fromFile ("data/words3.txt");

    String[] a = StdIn.readAllStrings();
    sort(a);
    show(a);

    // display results again using select
    StdOut.println();
    for (int i = 0; i < a.length; i++) {
      String ith = select(a, i);
      StdOut.println(ith);
    }

    COUNT_OPS = true;
    DoublingTest.run (20000, 5, N -> ArrayGenerator.integerRandomUnique (N),          (Integer[] x) -> sort (x));
    DoublingTest.run (20000, 5, N -> ArrayGenerator.integerRandom (N, 2),             (Integer[] x) -> sort (x));
    DoublingTest.run (20000, 5, N -> ArrayGenerator.integerPartiallySortedUnique (N), (Integer[] x) -> sort (x));
  }

}