数组 C++ 中的最长连续序列

Question

我必须创建一个函数来查找数组中最长的连续整数序列。

数组是这样的：

{1,2,3,4,4,4,5,7,9,10}

注意：跳过序列中的重复数字。

这是我的代码：

int longestSequence(const int arr[], int size)
{
  int currentSeq = 1;//set to 1 because all sequences are at least 1
  int maxSeq;

  for (int i = 1; i < size; i++)
  {
    if (arr[i] == arr[i-1]);//skip repeated numbers

    else if (arr[i-1] == (arr[i] - 1))//check if index is 1 greater than last
    {
      currentSeq++;
    }
    else //if not reset and hold last sequence value as max
      {
        maxSeq = currentSeq;
        currentSeq = 1;
      }
  }
  if (currentSeq > maxSeq) //if the last index of the array was last in the sequence
  {
    maxSeq = currentSeq;
  }

  return maxSeq;
}

我的代码一直为这个数组返回 2，但显然它应该是 5。

任何帮助将不胜感激。

Answer 1

您的数组中有 3 个序列：

• 1, 2, 3, 4, 4, 4, 5 有 5 个连续数字。
• 5, 7 不连续，将返回 1。
• 7, 9 也将返回 1。
• 9, 10 有 2 个连续，将返回 2 个。

在你的循环中你正在这样做：

for (int i = 1; i < size; i++)
{
    if (arr[i] == arr[i-1]);//skip repeated numbers

    else if (arr[i-1] == (arr[i] - 1))//check if index is 1 greater than last
    {
        currentSeq++;
    }
    else //if not reset and hold last sequence value as max
    {
        maxSeq = currentSeq; // <-- You're resetting your maxSequence even if
                             // currentSeq is smaller.
        currentSeq = 1;
    }
}

如下改变你的循环：

for (int i = 1; i < size; i++)
{
    if (arr[i] == arr[i-1])
        continue; //It is a good practice to skip the rest of the loop's 
                  //checks if you already know you're not gonna need them.
                  //The continue keyword skips only the current iteration of 
                  //the loop and improves intent readability.
    else if (arr[i-1] == (arr[i] - 1))//check if index is 1 greater than last
    {
        currentSeq++;
    }
    else //if not reset and hold last sequence value as max
    {
        currentSeq = 1; //This line stays.
                        //You only want to reset it under these specific conditions.
    }

    if (currentSeq > maxSeq) // Now you'll get the last sequence as well.
        maxSeq = currentSeq;
}

您可以在循环之外删除该检查并直接返回。如果最后一个currentSeq 大于maxSeq，则会正确注册。

此外，当我进行此更改时，我得到了一个编译错误，因为循环内的新if (currentSeq > maxSeq) 试图在设置之前读取maxSeq。所以将maxSeq的声明改为int maxSeq = 0。

通过这些更改，我得到了预期值 5。

Answer 2

通过使用您提供的示例数组在调试器中运行您的程序，我确定变量 currentSeq 确实达到了值 5，并且该值 5 正确写入了 maxSeq。但是，在程序的后期，maxSeq 会被值 1 和 2 覆盖。

在覆盖maxSeq 之前，您必须确定它是否已经包含比currentSeq 更高的值。在一种情况下，当您到达数组的末尾时，您已经这样做了。但在其他情况下，您不会这样做。

要使这样的比较起作用，您还必须初始化maxSeq，而不仅仅是currentSeq。否则，maxSeq 可能包含一个非常大的数字并且总是大于currentSeq。

Answer 3

#include <iostream>

void Log(int idx, int currentSeq, int maxSeq) {
  printf("current[%d] = %d, max[%d] = %d\n", idx, currentSeq, idx, maxSeq);
}

int LongestSequence(int* arr, int size) {
  int currentSeq = 1;
  int maxSeq = 1;  // max should be initialized as well
  Log(0, currentSeq, maxSeq);

  for (int i = 1; i < size; i++) {
    if (arr[i] == arr[i - 1]) {
    } else if (arr[i - 1] == (arr[i] - 1)) {
      currentSeq++;
    } else {
      currentSeq = 1;
    }
    // maxSeq should be updated in time, otherwise it would be tossed away
    maxSeq = std::max(maxSeq, currentSeq);
    Log(i, currentSeq, maxSeq);
  }

  return maxSeq;
}

int main() {
  int arr[] = {1, 2, 3, 4, 4, 4, 5, 7, 9, 10};
  std::cout << LongestSequence(arr, sizeof(arr) / sizeof(arr[0])) << std::endl;
}

输出：

current[0] = 1, max[0] = 1
current[1] = 2, max[1] = 2
current[2] = 3, max[2] = 3
current[3] = 4, max[3] = 4
current[4] = 4, max[4] = 4
current[5] = 4, max[5] = 4
current[6] = 5, max[6] = 5
current[7] = 1, max[7] = 5
current[8] = 1, max[8] = 5
current[9] = 2, max[9] = 5
5