public String truncateSentence(String s, int k) {

final StringBuilder result = new StringBuilder();

for (int index = 0, words = 0 ; index < s.length() && words < k ; index++) {

if (s.charAt(index) != ' ') {

result.append(s.charAt(index));

} else {

words++;

result.append(words < k ? ' ' : "");

}

}

return result.toString();

}