616. Add Bold Tag in String

class Solution {
    public String addBoldTag(String s, String[] dict) {
        int n = s.length();
        boolean[] f = new boolean[n];
        for(String d : dict)
        {
            int pos = s.indexOf(d);
            while(pos >= 0)
            {
                for(int i = pos; i < pos + d.length(); i++) f[i] = true;
                pos = s.indexOf(d, pos + 1);
            }
        }
        int i = 0, j = 0;
        StringBuilder sb = new StringBuilder();
        while(j < n)
        {
            if(!f[j]) sb.append(s.charAt(j++));
            else
            {
                i = j;
                while(j < n && f[j]) j++;
                sb.append("<b>").append(s.substring(i, j)).append("</b>");
            }
        }
        return sb.toString();
    }
}