滑动窗口，用双指针实现：

• 窗口内之和小于目标值时右指针向右移动增加窗口宽度；
• 窗口内之和大于目标值时左指针向右移动减少窗口宽度。

class Solution {
public:
    int minSubArrayLen(int target, vector<int>& nums) {
        if (nums.empty()) return 0;

        int left = 0;
        int curSum = 0;
        int minLength = INT_MAX;

        for (int right = 0; right < nums.size(); ++ right)
        {
            curSum += nums[right];
            while (left <= right && curSum >= target)
            {
                minLength = min(minLength, right - left + 1);
                curSum -= nums[left ++ ];
            }
        }

        return minLength == INT_MAX ? 0 : minLength;
    }
};