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use three-way partitioning to improve equal keys performance

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This commit is contained in:
Ahmet Inan 2024-03-10 12:23:16 +01:00
commit 1150d77a27
1 changed files with 18 additions and 13 deletions
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31
quick_select.hh
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@ -30,27 +30,32 @@ static inline void sort(TYPE *a, int i, int j, int k)
}
template <typename TYPE>
static int partition(TYPE *a, int l, int h)
static void partition(TYPE *a, int &l, int &h)
{
sort(a, (l + h) / 2, h, l);
for (int i = l; i < h; ++i)
if (a[i] < a[h])
swap(a, i, l++);
swap(a, l, h);
return l;
int m = l + (h - l) / 2;
sort(a, l, m, h);
TYPE pivot = a[m];
for (int i = l; i <= h;)
if (a[i] < pivot)
swap(a, i++, l++);
else if (a[i] > pivot)
swap(a, i, h--);
else
++i;
}
template <typename TYPE>
static TYPE select(TYPE *a, int l, int h, int k)
{
while (l < h) {
int i = partition(a, l, h);
if (k == i)
return a[k];
if (k < i)
h = i - 1;
int lt = l, gt = h;
partition(a, lt, gt);
if (k < lt)
h = lt - 1;
else if (k > gt)
l = gt + 1;
else
l = i + 1;
return a[k];
}
return a[l];
}