344.Reverse String

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Solution 1: accepted 17ms 3.83%

In your dream. 

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public String reverseString(String s) {
    StringBuilder sb = new StringBuilder();
    for (int i = s.length() - 1; i >= 0; i--) {
        sb.append(s.charAt(i) + ""); // charAt() is too expensive
    }
    return sb.toString();
}

Solution 2: accepted 4ms

Well, this is cheating XD. Try to imagine and implement what’s inside the function. 

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public String reverseString(String s) {
    StringBuilder sb = new StringBuilder(s);
    return sb.reverse().toString(); // reverse() has bitwise implementation so it's pretty fast
}

Solution 3: accepted 2ms 77.7%

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public String reverseString(String s) {
    // if (s.length() < 2) return s; // no impact on performance  
    char[] A = s.toCharArray(); // implementation: java.lang.System.arraycopy()
    int l = 0;
    int r = A.length - 1;
    while (l < r) {
        char temp = A[l];
        A[l] = A[r];
        A[r] = temp;
        l++;
        r--;
    }
    return String.valueOf(A);
}