Question

public static ArrayList<Integer> reverse (ArrayList<Integer> n) {
    ArrayList<Integer> result = new ArrayList<Integer>();

    for(int i = 0; i < n.size(); i++) {
        int j = n.size() - i - 1; 
        result.add(i, n.get(j));
    }

    return result;
}

but if i enter an array 1,2,3,4,5,6,7,8,9,10
the result is 10,9,8,7,6,5,4,3,2,10

Where is my mistake?

Answer 1

Actually, what you have there works fine. But you can simplify your for-loop by iterating ‘backwards’:

for(int i = n.size() - 1 ; i >= 0 ; i--)
    result.add(n.get(i));

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Oh, and one more thing I should mention. When you declare the list result, you might want to specify its capacity, since you know what it will be. i.e.:

ArrayList<Integer> result = new ArrayList<Integer>(n.size());