java: use StringBuilder to insert at the beginning

Question

I could only do this with String, for example:

String str = "";
for (int i = 0; i < 100; i++) {
    str = i + str;
}

Is there a way to achieve this with StringBuilder? Thanks.

Answer 1

// DO NOT DO THIS. It is quadratic-time!
StringBuilder sb = new StringBuilder();
for(int i=0;i<100;i++){
    sb.insert(0, Integer.toString(i));
}

Warning: It defeats the purpose of StringBuilder, but it does what you asked.

---

Better technique (although still not ideal):

1. Reverse each string you want to insert.
2. Append each string to a StringBuilder.
3. Reverse the entire StringBuilder when you're done.

This will turn an O(n²) solution into O(n).