原题链接：https://leetcode-cn.com/problems/maximum-difference-between-increasing-elements/

O(n)记录之前的最小值即可

class Solution {
public:
    int maximumDifference(vector<int>& nums) {
        int res = -1, mi = nums[0];
        int n = nums.size();
        for (int i = 1; i < n; ++i) {
            if (nums[i] > mi) {
                res = max(res, nums[i] - mi);
            } else {
                mi = nums[i];
            }
        }
        return res;
    }
};