Minimum Sum of Mountain Triplets I

Find indices i < j < k with nums[i] < nums[j] and nums[k] < nums[j] (so nums[j] is a "peak") and return the minimum possible sum nums[i]+nums[j]+nums[k], or -1 if none exists. Equivalent view: for each candidate peak j, pick the smallest qualifying left and right elements less than nums[j] to minimize the total.