放一个示例代码吧，题目是53. 最大子数组和

auto fast = []() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    return nullptr;
}();
class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        int res = INT_MIN;
        for(int i = 0, last = 0; i < nums.size(); i ++ ) {
            last = nums[i] + max(last, 0);
            res = max(res, last);
        }
        return res;
    }
};