Question

Possible Duplicate:
What is the fastest substring search algorithm?

How do I check if a string is present in a bigger string of length of 100,000 characters in C++ or Java?

I know a method str.find("sub_string"); but it can’t handle such a big string.
The max execution time is 1 sec.

Also the sub strings I need to look for can be 50,000!

Answer 1

This completes almost instantly (4ms) on my modest 1st gen Intel iMac. I put the search string between two blocks of 100,000 characters in case java searches backwards.

StringBuilder builder = new StringBuilder();
for (int i = 0; i < 100000; i++) {
    builder.append((char) i);
}
builder.append("sub_string");
for (int i = 0; i < 100000; i++) {
    builder.append((char) i);
}
String maxString = builder.toString();
long t1 = System.currentTimeMillis();
System.out.println(maxString.contains("sub_string"));
long t2 = System.currentTimeMillis();
System.out.println(t2 - t1);

Output

true
4