Optimizing Performance and Memory

Kis Solutions builds route-planning software for delivery fleets. One function summarizes a batch of delivery stops into a per-zone weight report, used to balance load across trucks. It works on small test batches, but on a real day's stop list it is both slow and uses far more memory than it should.

#include <vector>
#include <string>

struct DeliveryStop {
    std::string zoneCode;
    double weightKg;
};

std::vector<std::string> summarizeZones(std::vector<DeliveryStop> stops) {
    std::vector<std::string> zoneReport;

    for (int i = 0; i < stops.size(); i++) {
        bool alreadyListed = false;

        for (int j = 0; j < zoneReport.size(); j++) {
            std::string existingZone = zoneReport[j].substr(0, zoneReport[j].find(":"));
            if (existingZone == stops[i].zoneCode) {
                alreadyListed = true;
            }
        }

        if (!alreadyListed) {
            double totalWeight = 0;
            for (int k = 0; k < stops.size(); k++) {
                if (stops[k].zoneCode == stops[i].zoneCode) {
                    totalWeight += stops[k].weightKg;
                }
            }
            zoneReport.push_back(stops[i].zoneCode + ": " + std::to_string(totalWeight) + "kg");
        }
    }

    return zoneReport;
}

How would you optimize this function for better performance or lower memory usage? Explain each issue you find and exactly how your change fixes it.