Question

Suppose I have three lists:

list1 = a, c, d, r, t

list2 = a, d, t

list3 = a, r, d

Then list2 is contained in list1 but list3 is not because it is not in the same order.

I checked isSubCollection() from CollectionUtils in Apache Commons, and the containsAll() method, but it seems that they don’t consider order.

Answer 1

boolean isSubsequence(List<?> sup, List<?> sub) {
    int current = 0;
    for (Object obj: sup) {
      if (current == sub.size()) {
         return true;
      }
      if (obj.equals(sub.get(current)) {
        current++;
      }
    }
    return current == sub.size();
}

This algorithm is linear and requires only 1 iteration in sup list.

UPDATE
If you use linked lists, get operation may run in O(n). So you can use 2 iterators:

boolean isSubsequence(List<?> sup, List<?> sub) {
  Iterator<?> supIt = sup.iterator();
  for (Iterator<?> subIt = sub.iterator(); subIt.hasNext();) {
    Object current = subIt.next();
    boolean found = false;
    while (supIt.hasNext() && !found) {
      found |= supIt.next().equals(current);
    }
    if (!found) {
      return false;
    }
  }
  return true;
} 

But it looks uglier.