How about:
StringBuilder builder = new StringBuilder();
for(int i=99;i>=0;i--){
builder.append(Integer.toString(i));
}
builder.toString();
OR
StringBuilder builder = new StringBuilder();
for(int i=0;i<100;i++){
builder.insert(0, Integer.toString(i));
}
builder.toString();
But with this, you are making the operation O(N^2) instead of O(N).
Snippet from java docs:
Inserts the string representation of the Object argument into this character sequence. The overall effect is exactly as if the second argument were converted to a string by the method
String.valueOf(Object)
, and the characters of that string were then inserted into this character sequence at the indicated offset.