思路：
简单模拟
注意，nums[j]>nums[i]要保证

src：

class Solution:
    def maximumDifference(self, nums: List[int]) -> int:
        ans = -1
        n = len(nums)
        for i in range(n):
            for j in range(i + 1, n):
                if nums[j] > nums[i] and nums[j] - nums[i] > ans:
                    ans = nums[j] - nums[i]
        
        return ans