我一直在做學(xué)校作業(yè)。目標(biāo)：我正在提供一份超市顧客名單每個客戶都有一個郵政編碼和一個包含產(chǎn)品名稱的集合以及該客戶購買的這些產(chǎn)品的數(shù)量。我被要求返回一個地圖（Sting = zipCode，Product = Product），其中應(yīng)包含郵政編碼作為密鑰以及該郵政編碼最暢銷的產(chǎn)品。我得到的代碼：/** * (DIFFICULT!!!) * calculates a map of most bought products per zip code that is also ordered by zip code * if multiple products have the same maximum count, just pick one. * @return */public Map<String, Product> mostBoughtProductByZipCode() {    Map<String, Product> mostBought = null;    // TODO create an appropriate data structure for the mostBought and calculate its contents    return mostBought;}我一直在嘗試在地圖中使用地圖，但在實現(xiàn)這一點時遇到問題。這還遠(yuǎn)未完成，根本無法編譯。/** * (DIFFICULT!!!) * calculates a map of most bought products per zip code that is also ordered by zip code * if multiple products have the same maximum count, just pick one. * @return */public Map<String, Product> mostBoughtProductByZipCode() {    Map<String, Product> mostBought = null;    Map<String, Map<Product, Integer>> zipCodeProducts = new HashMap<>();    for (Customer customer : this.customers) {        String tmp = customer.getZipCode();        Map<Product, Integer> tmpMap = new HashMap<>();        for (Purchase purchase: customer.getItems()) {            tmpMap.put(purchase.getProduct(),purchase.getAmount());        }        if (!zipCodeProducts.containsKey(tmp)){            zipCodeProducts.put(tmp, tmpMap);        } else {            ???        }    }    // TODO create an appropriate data structure for the mostBought and calculate its contents    return mostBought;}我可以采取哪些步驟來修復(fù)此實施？我只是尋求提示而不是完整的解決方案。