Product of Array Except Self

Given an array of n integers where n > 1, nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].

Solve it without division and in O(n).

For example, given [1,2,3,4], return [24,12,8,6].

Follow up:
Could you solve it with constant space complexity? (Note: The output array does not count as extra space for the purpose of space complexity analysis.)

public class Solution {
    public int[] productExceptSelf(int[] nums) {
        int[] res = new int[nums.length];
        int mul = 1;
        for (int i = 0; i < nums.length; i++) {
            if (i == 0) {
                res[i] = 1;
            } else {
                res[i] = res[i - 1] * nums[i - 1];
            }
        }
        for (int i = nums.length - 1; i >= 0; i--) {
            if (i == nums.length - 1) {
                mul = 1;
            } else {
                mul = mul * nums[i + 1];
            }
            res[i] *= mul;
        }
        return res;
    }
}