Sum of Subarray Minimums

Problem:

You are given an array of integers, find the sum of all minimum values of every subarray of that given array.

size of array <= 3*10^4 >= elements of array

Example: nums = [2,1,3]

subarray     minimum value

2                2

2, 1            1

2, 1, 3        1

1                1

1, 3            1

3                3

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        sum = 9, that's the answer.