I want to get the pair of every two distinct element in set. I think that if using for-each loop I have to iterate with complexity of O(n^2). If using iterator, I can have two iterators where the second one points to the next of the first one, which means for the second loop I don't have to loop from the start. However, I can't seem to print my method correctly.
public static void main(String[] args){
Set<String> s = new HashSet<String>();
s.add("A");
s.add("B");
s.add("C");
s.add("D");
Iterator<String> itr1 = s.iterator();
while (itr1.hasNext()){
Iterator<String> itr2 = itr1;
String s1 = itr1.next();
while (itr2.hasNext()){
String s2 = itr2.next();
System.out.println(s1 + " " + s2);
}
}
}
The output is
A B
A C
A D
However what I want is:
A B
A C
A D
B C
B D
C D
I don't think that
Iterator<String> itr2 = itr1;
does what you think it does. It means that itr2 is literally the same object as itr1. It is not a deep or stateful copy.
This would be easier if you were working with a list, since we could rely on indices for order. In order to use Iterator and Set, you'll need to maintain a collection of objects that you've already used:
Iterator<String> itr1 = s.iterator();
Set<String> used = new HashSet<>(); // track the elements that have been used in the first column
while (itr1.hasNext()) {
Iterator<String> itr2 = s.iterator(); // a new iterator
String s1 = itr1.next();
used.add(s1); // track what we've used
while (itr2.hasNext()) {
String s2 = itr2.next();
if (used.contains(s2));
continue; // we've alread used s2
System.out.println(s1 + " " + s2);
}
}
Theoretically, it would be better to use an array (or list) and for loop to do what you want to do:
String[] elements = s.toArray(new String[s.size()]);
for (int i = 0; i < elements.length; ++i) {
String s1 = elements[i];
// loop through all successive elements
for (int j = i + 1; j < elements.length; ++j) {
String s2 = elements[j];
System.out.println(s1 + " " + s2);
}
}