为什么我的递归最长递增子序列函数不适用于大输入？

Question

I wrote it to work on a single test case at a time. It either takes too long on online judges or returns wrong answers

Source: The problem I used to test it on

It works perfectly for small cases:

#include <iostream>
  #include <algorithm>
  #include <vector>
  int LIS[100000];
  void LS (int *arr , int n)
  {
      if (n == 0)
      {
          LIS[0] = 1;
          return;
      }
      if (LIS[n])
      {
          return;
      }
      int i = 0;
      int max = 0;
      while (i < n)
      {
          if (arr[i] < arr[n])
          {
              LS(arr,i);
              if (LIS[i] + 1 > max)
              {
                  max = 1 + LIS[i];
              }
          }
          ++i;
      }
      LIS[n] = max;

  }
  int main()
  {
      int n;
      std::cin >> n;
      int arr[n];
      for(int i = 0 ; i < n ; ++i) std::cin >> arr[i];
      LS(arr,n-1);
      std::sort (LIS , LIS+n);
      std::cout << "\n" << LIS[n-1] << "\n";
  }

Answer 1

You said it works perfectly small cases.. than maybe it is stack overflow..

A function call consume stack memory..

If recursive call depth is too deep, stack memory runs out, and crash..

Answer 2

When you read int n value from std::cin , you have to dynamically allocate memory for your array arr , so you should first declare int *arr , then get the user input the same way as you're doing it now, and then allocate memory using arr = new int[n] .

When it comes to the long time it takes to compute next values using recursive function, you should think about remaking the function to use tail recursion, which is much closer to iteration. You can check the difference by writing two programs for counting Fibonacci numbers - one recursive and another one iterative, then check how long it takes to compute ~50th value using both methods.