831. Masking Personal Information(#1 ).java

class Solution {
    public String maskPII(String S) {
        StringBuilder res = new StringBuilder();
        int idx = S.indexOf('@');
        int len = S.length();
        if (idx > 0) {
            res.append(Character.toLowerCase(S.charAt(0)));
            res.append("*****");
            res.append(S.substring(idx - 1).toLowerCase());
        } else {
            int lastFour = 0;
            for (int i = len - 1; i >= 0; i--) {
                if (Character.isDigit(S.charAt(i))) {
                    if (lastFour < 4) {
                        res.insert(0, S.charAt(i));
                        lastFour++;
                    } else {
                        res.insert(0, '*');
                    }
                }
            }
            int cntOfChar = res.length();
            if (cntOfChar - 10 > 0) { 
                res.insert(0, '+');
                cntOfChar += 1;
                res.insert(cntOfChar - 10, '-');
                cntOfChar += 1;
            }
            res.insert(cntOfChar - 4, '-');
            res.insert(cntOfChar - 7, '-');
        }
        return res.toString();
    }
}