Input: cost = [10, 15, 20]
Explanation: Cheapest is start on cost[1], pay that cost and go to the top.
Input: cost = [1, 100, 1, 1, 1, 100, 1, 1, 100, 1]
Explanation: Cheapest is start on cost[0], and only step on 1s, skipping cost[3].
1. cost will have a length in the range [2, 1000].
2.Every cost[i] will be an integer in the range [0, 999].