天池 在线编程 所有子数组之和（排列组合）

示例
示例1:
输入: nums = [1, 2, 3]
输出: 20
解释: {1} + {2} + {3} + {2 + 3} + {1 + 2} + {1 + 2 + 3} = 20

示例2
输入: [1, 2]
输出: 6
解释: {1} + {2} + {1, 2} = 6 

class Solution {
public:
    /**
     * @param nums: a Integer list
     * @return: return the sum of subarrays
     */
    int SubArraySum(vector<int> &nums) {
        // write your code here
        int ans = 0, n = nums.size();
        vector<int> dp(n, 0);
        dp[0] = nums[0]; // 前n个数的所有子数组的和
        for(int i = 1; i < nums.size(); ++i)
        {
        	int k = i+1;
        	dp[i] = dp[i-1];
        	while(k)
        	{
        		dp[i] += nums[k-1]*k;
        		k--; 
        	}
        }
        return dp[n-1];
    }
};

class Solution {
public:
    /**
     * @param nums: a Integer list
     * @return: return the sum of subarrays
     */
    int SubArraySum(vector<int> &nums) {
        // write your code here
        int ans = 0, n = nums.size();
        for(int i = 0; i < nums.size(); ++i)
        {
        	ans += nums[i]*(i+1)*(n-i);
        }
        return ans;
    }
};