C++ 递归来决定数组中的两个值

Question

I am trying to create a function that can increment either one or two moves ahead (whichever index contains a smaller integer) in an array to get to the last value using recursion. This is for an assignment, and I have been working for over a day to try and figure this out. So I am not asking for the actual code to do this, but am looking for what's wrong in the code I have so far. I put together a function that works, but does it incrementally.

int findLowestSum(int arr[], int low, int high, int arrSize) {
    int minVal = 0;
    while (high < arrSize) {

        if (arr[low] < arr[high]) {
            minVal += arr[low];
            low += 1;
            high = low +1;
        }
        else {
            minVal += arr[high];
            low +=2;
            high = low +1;

        }
    }
    return minVal;
}

Here is what I have so far for the recursive version. Without trying to make a bad recursion joke, I have been going around in circles on this for about a day.

int findLowestSum(int arr[], int startIdx, int endIdx) {
    int minVal = 0;
    int res1, res2;
    int sum = 0;


    if (endIdx - startIdx <= 3)
        return sum += arr[endIdx];
    else {
        if (arr[startIdx+1] < arr[startIdx + 2]) {

            sum+= arr[startIdx+1];
            res1 = findLowestSum(arr, startIdx + 1, endIdx);


        }
        else {
            sum+= arr[startIdx+2];
            res2 = findLowestSum(arr, startIdx + 2, endIdx);

        }

    return sum + arr[endIdx];

    }


}

Any insight into what is actually happening with my recursive attempt, and where i need to fix would be greatly appreciated.

Answer 1

Try thinking with iterators instead of array indexes. Then the solution becomes trivial:

#include <iterator>

int findLowestSum(int const* begin, int const* const end) {
  if (begin >= end) return 0; // std::next(begin) would be UB
  auto const next = std::next(begin);
  if (next >= end) return 0;
  if (*begin >= *next) {
    begin = next;
  }
  return *begin + findLowestSum(std::next(begin),end);
}

This can be called with findLowestSum(std::begin(arr),std::end(arr)) .

You only need the <iterator> header for std::next , std::begin and std::end . You can also rewrite this if you don't want the <iterator> dependency.