【专题】 考虑每一项对结果的贡献

A common problem is to give you an array, and then give you an index, and ask you to find the sum of that index. For example, give you an array, give you an index for the maximum value of a subarray, and ask you to find the sum of all the maximum values of the subarrays. We can consider the contribution of each item in the array, and then add it up.