欧拉计划第13题：百位50位数字之和前十位——代码问题排查

完全理解你这种明明有思路但代码跑不起来的困惑！我帮你梳理下这段Java代码里的问题：

你提到尝试解决欧拉计划第13题——计算100个50位数字之和的前十位，虽然已经用其他方法搞定，但还是纠结这段代码为啥不行。先把你的代码贴出来（修正了转义符号）：

import java.util.ArrayList;

public class LargeSum {
    public static ArrayList<String> addAll(ArrayList<ArrayList<Integer>> listOfResults) {
        ArrayList<String> result = new ArrayList<String>();
        int singlesum = 0;
        int remainder = 0;
        for (int i = 49; i >= 0; i--) {
            System.out.println(remainder + "to rem");
            singlesum += remainder;
            for (ArrayList<Integer> list : listOfResults) {
                System.out.println(list.get(i));
                singlesum += list.get(i);
            }
            String lastChar = String.valueOf(singlesum);
            singlesum = 0;
            result.add(String.valueOf(lastChar.charAt(lastChar.length() - 1)));
            remainder = 0;
            if (lastChar.length() > 1) {
                remainder = Integer.parseInt(lastChar.substring(0, lastChar.length() - 1));
            }
        }
        if (remainder > 0) result.add(String.valueOf(remainder));
        return result;
    }

    public static ArrayList<ArrayList<Integer>> createLists() {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        String c = "37107287533902102798797998220837590246510135740250" +
                "46376937677490009712648124896970078050417018260538" +
                "74324986199524741059474233309513058123726617309629" +
                "91942213363574161572522430563301811072406154908250" +
                "23067588207539346171171980310421047513778063246676" +
                "89261670696623633820136378418383684178734361726757" +
                "28112879812849979408065481931592621691275889832738" +
                "44274228917432520321923589422876796487670272189318" +
                "47451445736001306439091167216856844588711603153276" +
                "70386486105843025439939619828917593665686757934951" +
                "62176457141856560629502157223196586755079324193331" +
                "64906352462741904929101432445813822663347944758178" +
                "92575867718337217661963751590579239728245598838407" +
                "58203565325359399008402633568948830189458628227828" +
                "80181199384826282014278194139940567587151170094390" +
                "35398664372827112653829987240784473053190104293586" +
                "86515506006295864861532075273371959191420517255829" +
                "71693888707715466499115593487603532921714970056938" +
                "54370070576826684624621495650076471787294438377604" +
                "53282654108756828443191190634694037855217779295145" +
                "36123272525000296071075082563815656710885258350721" +
                "45876576172410976447339110607218265236877223636045" +
                "17423706905851860660448207621209813287860733969412" +
                "81142660418086830619328460811191061556940512689692" +
                "51934325451728388641918047049293215058642563049483" +
                "62467221648435076201727918039944693004732956340691" +
                "15732444386908125794514089057706229429197107928209" +
                "55037687525678773091862540744969844508330393682126" +
                "18336384825330154686196124348767681297534375946515" +
                "80386287592878490201521685554828717201219257766954" +
                "78182833757993103614740356856449095527097864797581" +
                "16726320100436897842553539920931837441497806860984" +
                "48403098129077791799088218795327364475675590848030" +
                "87086987551392711854517078544161852424320693150332" +
                "59959406895756536782107074926966537676326235447210" +
                "69793950679652694742597709739166693763042633987085" +
                "41052684708299085211399427365734116182760315001271" +
                "65378607361501080857009149939512557028198746004375" +
                "35829035317434717326932123578154982629742552737307" +
                "94953759765105305946966067683156574377167401875275" +
                "88902802571733229619176668713819931811048770190271" +
                "25267680276078003013678680992525463401061632866526" +
                "36270218540497705585629946580636237993140746255962" +
                "24074486908231174977792365466257246923322810917141" +
                "91430288197103288597806669760892938638285025333403" +
                "34413065578016127815921815005561868836468420090470" +
                "23053081172816430487623791969842487255036638784583" +
                "11487696932154902810424020138335124462181441773470" +
                "63783299490636259666498587618221225225512486764533" +
                "67720186971698544312419572409913959008952310058822" +
                "95548255300263520781532296796249481641953868218774" +
                "76085327132285723110424803456124867697064507995236" +
                "37774242535411291684276865538926205024910326572967" +
                "23701913275725675285653248258265463092207058596522" +
                "29798860272258331913126375147341994889534765745501" +
                "18495701454879288984856827726077713721403798879715" +
                "38298203783031473527721580348144513491373226651381" +
                "34829543829199918180278916522431027392251122869539" +
                "40957953066405232632538044100059654939159879593635" +
                "29746152185502371307642255121183693803580388584903" +
                "41698116222072977186158236678424689157993532961922" +
                "62467957194401269043877107275048102390895523597";
        return results;
    }
}

核心问题所在 #

1. createLists()方法完全没做数据拆分工作
   你初始化了results列表，但根本没把长字符串c拆分成100个50位的数字，更没把每个数字的每一位转成Integer存入子列表。这就导致addAll()方法接收的listOfResults是空的，遍历的时候没有任何元素可以累加，自然得不到正确结果，甚至可能抛出空指针相关异常。

2. 字符串c的长度可能有问题
   欧拉计划第13题的输入是刚好100个50位数字，总长度应该是5000位。你可以检查下拼接后的c长度是否达标，要是少了几位，后续拆分也会出错。

3. 结果处理的小细节
   就算前面的问题都解决了，addAll()方法返回的result列表是从个位到高位存储的数字字符，要得到前十位的话，还需要把列表反转，再取前10个字符拼接。

修复后的代码示例 #

我帮你完善了createLists()方法，同时补充了主方法来测试：

import java.util.ArrayList;
import java.util.Collections;

public class LargeSum {
    public static ArrayList<String> addAll(ArrayList