快排找数组中第k小元素

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public kth{
  public static Comparable select(Comparable[] a, int k){
    StdRandom.shuffer(a);
    int lo = 0;
    int hi = a.length-1;
    while (hi > lo){
      int j = partition(a,lo,hi);
      if (j == k) return a[k];
      else if (j > k) hi = j - 1;
      else lo = j + 1;
    }
    return a[k];
  }
  
  int patition(Comparable[] a, int lo, int hi){
    int i = lo;
    int j = hi;
    Comparable v = a[lo];
    while (true){
      while (a[++i] < v) if (i == hi) break;
      while (a[--j] > v) if (j == lo) break;
      if (i >= j) break;
      exc(a,i,j);
    }
    exc(a,lo,j);
    return j;
  }
  
  void exc (Comparable[] a, int i, int j){
    Comparable t = a[i];
    a[i] = a[j];
    a[j] = t;
  }
}