Solution.cpp

class Solution {
public:
    int kthSmallestSubarraySum(vector<int>& nums, int k) {
        int l = 1 << 30, r = 0;
        for (int& x : nums) {
            l = min(l, x);
            r += x;
        }
        auto f = [&](int s) {
            int cnt = 0, t = 0;
            for (int i = 0, j = 0; i < nums.size(); ++i) {
                t += nums[i];
                while (t > s) {
                    t -= nums[j++];
                }
                cnt += i - j + 1;
            }
            return cnt;
        };
        while (l < r) {
            int mid = (l + r) >> 1;
            if (f(mid) >= k) {
                r = mid;
            } else {
                l = mid + 1;
            }
        }
        return l;
    }
};