238. 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.)

2.思路：
O(1)Space：算left的时候可以直接放到结果中去，算right的时候其实只需要right[i+1]的值，因此用一个变量即可

   public int[] productExceptSelf(int[] nums) {  
     int[] ans = new int[nums.length];  
     if(nums.length == 0) return ans;  
     Arrays.fill(ans,1);  
     for(int i = 1; i < nums.length; i++){  
       ans[i] = nums[i-1]*ans[i-1];  
     }     
     int right = 1;  
     for(int i = nums.length-2; i >= 0; i--){  
       right = nums[i+1]*right;  
       ans[i] = ans[i] *right;  
     }  
     return ans;  
   }