Removing ALL Duplicates on a List

Question

I'm facing a Requirement Specs that can be simplified to this scenario. Simply put, the specs say to remove all duplicates from a list. This means not to leave one duplicate record from the list but to remove ALL duplicate records.

Example:

List01: a, b, c, d, c, a

Expected Result: List01: b, d

Since a and c were duplicates, they were removed from the list.

I've been struggling on this for quite a while now.

All help is appreciated. Thanks.

Answer 1

In this case, you can use two sets to find the duplicates:

Set<String> dup1 = new Set<String>(), dup2 = new Set<String>();
for(String item: stringList) {
  if(!dup1.add(item)) { // Have we seen this before?
    dup2.add(item);     // Yes, it's a duplicate
  }
}
Integer index;
for(String dup: dup2) { // For each duplicate
  while((index = stringList.indexOf(dup)) > -1) { // If exists in list
    stringList.remove(index);  // Remove the value
  }
}

If the original list order doesn't matter, you can perform the removals even faster:

Set<String> dup1 = new Set<String>(), dup2 = new Set<String>();
for(String item: stringList) {
  if(!dup1.add(item)) { 
    dup2.add(item);
  }
}
dup1.removeAll(dup2); // Remove all duplicates
stringList.clear();   // Clear original list
StringList.addAll(dup1); // And put the surviving items back in