据说华为的一道面试题

// from http://www.robberphex.com/interview-question-from-huawei/
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<Integer, List<List<Integer>>> sumCount = new HashMap<>();
        Map<Integer, List<List<Integer>>> prodCount = new HashMap<>();
        Set<List<Integer>> candidates = new HashSet<>();

        for (int i = 1; i <= 20; i++) {
            for (int j = i + 1; j <= 20; j++) {
                // 第一步，A猜不出来
                if (i + j < 4 || i + j > 38) {
                    continue;
                }

                List<List<Integer>> sumCnt = sumCount.getOrDefault(i + j, new ArrayList<>());
                sumCnt.add(Arrays.asList(i, j));
                sumCount.put(i + j, sumCnt);

                List<List<Integer>> prodCnt = prodCount.getOrDefault(i * j, new ArrayList<>());
                prodCnt.add(Arrays.asList(i, j));
                prodCount.put(i * j, prodCnt);
            }
        }

        for (Map.Entry<Integer, List<List<Integer>>> entry : sumCount.entrySet()) {
            // 第二步，B猜不出来
            int cnt = 0;
            List<Integer> res = null;
            if (entry.getValue().size() > 1) {
                for (List<Integer> list : entry.getValue()) {
                    if (prodCount.get(list.get(0) * list.get(1)).size() > 1) {
                        res = list;
                        cnt++;
                    }
                }
            }
            // 第三步，A猜出来了
            if (cnt == 1) {
                candidates.add(res);
            }
        }

        //第四步，从第三步拿到的candidates中，过滤
        for (List<Integer> num : candidates) {
            int poss = 0;
            for (List<Integer> prodNum : prodCount.get(num.get(0) * num.get(1))) {
                int cnt = 0;
                for (List<Integer> sumNum : sumCount.get(prodNum.get(0) + prodNum.get(1))) {
                    if (prodCount.get(sumNum.get(0) * sumNum.get(1)).size() > 1) {
                        cnt++;
                    }
                }
                if (cnt == 1) {
                    poss++;
                }
            }
            if (poss == 1) {
                System.out.println(num);
            }
        }
    }
}