int substrCount(String S[0...n-1]){
    int count = 0;
    for (int i = 0; i<=n-2; i++) {
        for (int j=i+1; j<i+2; j++) {
            if ((S[i] == 'B' && S[j] == 'C' ) || (S[i] == 'C' && S[j] == 'B')) {
                count = count + 1;
            }
        }
    }
}

static int substrCount( String str) {
    int count=0;
    for (int i=0; i<str.length()-1; i++)
    {
        boolean bc = (str.charAt(i) == 'B' && str.charAt(i+1) == 'C');
        boolean cb = (str.charAt(i) == 'C' && str.charAt(i+1) == 'B');
        if (bc || cb) {
            count++;
        }
    }
    return count;
}

"ACBAA" gives result 1
"ABCBA" gives result 2
"BCBCB" gives result 4
"BBBBB" gives result 0