Java 将列切换为行

问题描述

现在我想将列转换为行，例如以下示例： 有一些列表：

List1: {"1","2","3"}
List2: {"1","3","4"}
List3: {"1","4","5"}

然后我想将它们转换为：

ListA: {"1","1","1"}
ListB: {"2","2"}
ListC: {"3","3"}
ListD: {"","4"}
ListE: {"","","5"}

以下是我的解决方案：

List<List<String>> list = Lists.newArrayList(
                Lists.newArrayList("1","5"),Lists.newArrayList("1","4"),"3"));
List<List<String>> res = Lists.newArrayList();
TreeMap<Integer,List<String>> map = list.stream().collect(Collectors.toMap(List::size,Function.identity(),(b1,b2) -> b1,TreeMap::new));
int size = map.get(map.lastKey()).size();
for (int i = 0; i < size; i++) {
    int finalI = i;
    res.add(list.stream().map(list1 -> {
        if (list1.size() > finalI) {
            return list1.get(finalI);
        }
        return "";
    }).collect(Collectors.toList()));
}

但是它的复杂度是 O(n²)，还有其他更好的解决方案吗？

解决方法

public static void main(String[] args) {
    // Create input list
    List<String> list1 = List.of("1","2","3");
    List<String> list2 = List.of("1","3","4");
    List<String> list3 = List.of("1","4","5");

    // Create ouput list
    List<List<String>> outputList = new ArrayList<>();

    // Iterate for max length of lists
    IntStream
        .range(0,getMax(list1,list2,list3))
        .forEach(index -> {
            outputList.add(
                Arrays.asList(getElementByIndexOrEmpty(list1,index),getElementByIndexOrEmpty(list2,getElementByIndexOrEmpty(list3,index))
            );
        });

    System.out.println(outputList);
}

// Return element at index if present else return empty string
private static String getElementByIndexOrEmpty(List<String> list,int index) {
    return list.size() > index ? list.get(index) : "";
}

// Find max length of lists
private static int getMax(List<String> list1,List<String> list2,List<String> list3) {
    return Math.max(list1.size(),Math.max(list2.size(),list3.size()));
}

代码只遍历所有列表一次（对于任何列表的最大长度）。