Split Array with Equal Sum — C++

// Time:  O(n^2)
// Space: O(n)

class Solution {
public:
    bool splitArray(vector<int>& nums) {
        if (nums.size() < 7) {
            return false;
        }
        
        vector<int> sum(nums.size());
        sum[0] = nums[0];
        for (int i = 1; i < nums.size(); ++i) {
            sum[i] = sum[i - 1] + nums[i];
        }
        for (int j = 3; j < nums.size() - 3; ++j) {
            unordered_set<int> lookup;
            for (int i = 1; i < j - 1; ++i) {
                if (sum[i - 1] == sum[j - 1] - sum[i]) {
                    lookup.emplace(sum[i - 1]);
                }
            }
            for (int k = j + 2; k < nums.size() - 1; ++k) {
                if (sum[nums.size() - 1] - sum[k] == sum[k - 1] - sum[j] &&
                    lookup.count(sum[k - 1] - sum[j])) {
                    return true;
                }
            }
        }
        return false;
    }
};