Reversing an Array

Just push each item onto the stack and then pop it back off. Because of the stack's LIFO nature, the items come back out in reverse order.

int[] reverserArray(int[] arr) {
    Stack<Integer> stack = new Stack<>();
    
    for(int i = 0; i < arr.length; ++i) {
        stack.push(arr[i]);    
    }
    
    for(int i = 0; !stack.isEmpty(); ++i) {
        arr[i] = stack.pop();
    }
    
    return arr;
}

Runtime : we do two iteration over the array$\text{O}(n)$