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.)

Solution

• 先o(n)　时间累乘nums[i]左边
• 再O(n) 累乘nums[i]右边

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public class Solution {
    public int[] productExceptSelf(int[] nums) {
        int n = nums.length;
        int[] ret = new int[n];
        //累乘nums[i]左面的
        ret[0] = 1;
        for (int i=1;i<n;i++){
            ret[i] = nums[i-1]*ret[i-1];
        }

        // 类成nums[i]右边的
        int right = 1;
        for (int i=n-1;i>=0;i--){
            ret[i] *= right;
            right *= nums[i];
        }
        return ret;
    }
}