leetcode review （2）(Data structure) (Java)

There are some useful data structure in Java:
Array
If it is a sorted array, we can use Arrays.binarySearch(array, num)

BigInteger
If we need to process very big number, we can use BigInteger.
import java.math.BigInteger
x2 = x2.add(x1); // plus
x1 = x2.subtract(x2); // minus

String
s1.startsWith(s2, index);

Something I need to pay attention to:

1. (int)(num%137) != (int)curr%137