解法一

class Solution {
    public String defangIPaddr(String address) {
        String ans = "";
        for(int i = 0 ; i < address.length();i++){
            if(address.charAt(i)=='.') {
                ans += "[.]";
            }
            else ans+= address.charAt(i);
        }
        return ans;
    }
}

解法二

class Solution {
    public String defangIPaddr(String address) {
        String result = "";
        int acount = 0;
        for (int i = 0; i < address.length(); i++) {
            if (address.charAt(i) == '.') {
                result += address.substring(acount, i);
                result += "[.]";
                acount = i + 1;
            }
        }
        return result + address.substring(acount, address.length());
    }
}