Cancel out the doubled keystrokes
A faulty keyboard doubles a letter now and then. The cleaner removes any two identical letters sitting next to each other, and keeps going while that leaves a new pair behind.
- Two identical characters next to each other both disappear.
- Removing a pair can bring two more together, and those go too.
- Keep going until no adjacent pair is left.
- Text with nothing to remove comes back unchanged.
collapseDoubles(typed: string) → string
Java needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
String collapseDoubles(String typed) {
}Worked examples
| Call | Result |
|---|---|
collapseDoubles("abbaca") | "ca" |
collapseDoubles("azxxzy") | "ay" |
collapseDoubles("abc") | "abc" |
collapseDoubles("aa") | "" |
Hint
Build the result on a stack. For each character, either it cancels the one on top or it goes on top — no rescanning needed.
Reference solution in Java
String collapseDoubles(String typed) {
StringBuilder kept = new StringBuilder();
for (int i = 0; i < typed.length(); i++) {
char ch = typed.charAt(i);
int last = kept.length() - 1;
if (last >= 0 && kept.charAt(last) == ch) kept.deleteCharAt(last);
else kept.append(ch);
}
return kept.toString();
}