public void quicksort(int a[ ]) {
StackhPair <Integer ,Integer> = stack = new Stack<>();
stack.push(new Pair hInteger ,Integer i(1, a.length − 1));
int min = 0;
for(int i = 1; i < a.length; i++) if(a[i] < a[min]) min = i;
int t = a[0]; a[0] = a[min]; a[min] = t;
while(!stack.isempty()) {
Pair hInteger ,Integer i p = stack.pop();
int l = p.first(), r = p.second();
int i = l − 1, j = r , pivot = j;
do {
do { i++; } while(a[i] < a[pivot]);
do { j−−; } while(a[j] > a[pivot]);
t = a[i]; a[i] = a[j]; a[j] = t;
} while(i < j);
a[j] = a[i]; a[i] = a[r ]; a[r ] = t;
if(r − i > 1) stack.push(new Pair hInteger ,Integer i(i + 1, r ));
if(i − l > 1) stack.push(new Pair hInteger ,Integer i(l, i − 1));
}
}