LeetCode 4Sum, TLE，怎么破？

4sum.cpp

// LeetCode, 4Sum
// 先排序，然后二分查找
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& num, int target) {
        vector<vector<int>> result;
        if (num.size() < 4) return result;
        sort(num.begin(), num.end());

        auto last = num.end();
        for (auto a = num.begin(); a < prev(last, 3);
                a = upper_bound(a, prev(last, 3), *a)) {
            for (auto b = next(a); b < prev(last, 2);
                    b = upper_bound(b, prev(last, 2), *b)) {
                for (auto c = next(b); c < prev(last);
                        c = upper_bound(c, prev(last), *c)) {
                    const int d = target - *a - *b - *c;

                    if (binary_search(next(c), last, d))
                        result.push_back(vector<int> { *a, *b, *c, d });
                }
            }
        }

        return result;
    }
};