How to reverse a String? How to deal with it if the input string is null?

Question

Peanut Elbow · Accepted Answer

If you want to save on the extra "swap" memory, you can use bitwise operator XOR like this:

public String reverse( String string ) {
 
    byte[] array = string.getBytes();
 
    for( int i = 0, j = array.length - 1; i < array.length / 2; i++, j-- ) {
 
        array[ i ] ^= array[ j ];
        array[ j ] ^= array[ i ];
        array[ i ] ^= array[ j ];
    }
 
    return new String( array );
}

Sigiloso · Answer

string a="";
    for(int i=a.length()-1; i>=0; i--){
        str+=a.charAt(a);
    }
    return str;
}

Peanut Elbow · Answer

The above approach becomes O(N^2)  due to use of  += inside a for loop. Every time we use +=, a new string is created from the copied over contents of the last string to which another string is appended. On an average there are N/2 copy operations.

This can be bettered using StringBuilder like this:
StringBuffer strBuffer = new StringBuffer();
    for(int i=a.length()-1; i>=0; i--){
        strBuffer.append(a.charAt(a));
    }
    return strBuffer.toString();
}

However, this can be improvised even more using the just N/2 iterations (instead of N above) :

public String reverse( String string ) {
 
    byte[] array = string.getBytes();
    byte swap;
 
    for( int i = 0, j = array.length - 1; i < array.length / 2; i++, j-- ) {
        swap = array[ j ];
        array[ j ] = array[ i ];
        array[ i ] = swap;
    }
 
    return new String( array );
}

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