How to Count Binary Substrings in Java

The challenge

Give a string s, count the number of non-empty (contiguous) substrings that have the same number of 0’s and 1’s, and all the 0’s and all the 1’s in these substrings are grouped consecutively.

Substrings that occur multiple times are counted the number of times they occur.

Example 1:

Input: "00110011"
Output: 6
Explanation: There are 6 substrings that have equal number of consecutive 1's and 0's: "0011", "01", "1100", "10", "0011", and "01".

Notice that some of these substrings repeat and are counted the number of times they occur.

Also, "00110011" is not a valid substring because all the 0's (and 1's) are not grouped together.

Example 2:

Input: "10101"
Output: 4
Explanation: There are 4 substrings: "10", "01", "10", "01" that have equal number of consecutive 1's and 0's.

Note:s.length will be between 1 and 50,000.s will only consist of “0” or “1” characters.

The solution in Java code

class Solution {
    public int countBinarySubstrings(String s) {
        int arr [] = new int[2];
        int res = 0;
        if (s.charAt(0) == '0') arr[0]++;
        else arr[1]++;
        for (int i = 1; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch != s.charAt(i-1)) arr[ch - '0'] = 1;
            else arr[ch - '0']++;
            if (arr[1 - (ch - '0')] > 0) {
                arr[1 - (ch - '0')]--;
                res++;
            }
        }
        return res;
    }
}