What is the complexity of this simple piece of code?

Question

I\'m pasting this text from an ebook I have. It says the complexity if O(n²) and also gives an explanation for it, but I fail to see how.

Question: What i

Answer 1

There's a typo in this book.

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1st case :

public String makeSentence(String[] words) {
    String sentence = new String();
    for (String w : words) sentence += w;
    return sentence;
}

Complexity : O(n^2) -> (n words) x (n characters copied at each iteration, for copying the current sentence into a StringBuffer)

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2nd case :

public String makeSentence(String[] words) {
    StringBuffer sentence = new StringBuffer();
    for (String w : words) sentence.append(w);
    return sentence.toString();
}

Complexity : O(n) -> (n words) x O(1) (amortized complexity for StringBuffer concatenation)