Using only if-else statements to find specific element in an array with length n and elements from 0 to n-1

Question

I'm stuck in solving an interview question. The goal is to find a specific element from an array with unknown length (cannot use.length) and return the number of steps, but for an array with a length of n, the elements are guaranteed to be from 0 to n-1, no duplicates. For example, if the array's length is 5, the elements are {0, 1, 2, 3, 4} but the order may be different. Additional requirements are no loops, no static/global variables, and no helper functions, and the only parameters passing in are the array int[] arr and the target value int x, no extra parameters allowed, the array remains the same after all the operations have done.

//So you can only write in the body of the following method, no outside variables nor methods could be used.

private int findElement (int[] arr, int x) {

}

What I have gotten so far is, since the elements are guaranteed to be 0 to n-1 , I can use the target number as an index and go back to the array to see if the number arr[x] equals the number x I want. If not, I take that number arr[x] and make it my new index, repeating until I find the target value.

int[] arr = {4, 1, 0, 2, 3}
int target = 3;
arr[3] = 2; //take target as the initial index
arr[2] = 0;
arr[0] = 4;
arr[4] = 3; //we got the number we want
//steps total is 3 since the question says the first time doesn't count.

Question: I tried to solve this by recursion, but since I am always comparing the following values with the initial parameter value, in the above case I always wanted to find 3. So how to store that information without static variables or extra parameters is my bigges problem. Is there any other way I can store the initial parameter value and pass it through the whole process?

    private int findElement(int [] arr, int x) {
        int actualN = arr[x];
        if (actualN == **???**) { //can't be x cuz x is changing but I always want 3
            return 0;
        } else {
            return findElement(arr, arr[x]) + 1;
        }

    }

Preferably using Java Any hints or help would be greatly appreciated.

Answer 1

You were almost there

    // t is the target number, o is teh array offset
    static int find(int [] arr, int t, int o) {
        if (arr[o] == t)
            return o;
       return  find(arr, t, o + 1);
    }

and

    static void Main(string[] args) {
        int[] arr = { 4, 1, 0, 2, 3 };
        int target = 3;
        int x = find(arr, 3, 0);
   }

---

if only 2 args allowed - I missed that

in c

static int* find(int* arr, int t) {
    if (*arr == t)
        return arr;
    return find(arr + 1, t);

}

int main() {

    int arr[] = {4, 1, 0, 2, 3};
    int target = 2;
    int x = find(arr, target) - arr;
}

in c#

    static unsafe int* find(int * arr,  int t) {
        if (*arr == t)
            return arr;
       return find(arr + 1,t);
    }

    static void Main(string[] args) {
        int[] arr = { 4, 1, 0, 2, 3 };
        int target = 3;
        unsafe {
            fixed (int * p = &arr[0]) {
                int x = (int)(find(p, target) - p);
            }
        }
    }

Answer 2

Probably this should work:

private int findElement(int [] arr, int x) {
        int currValue = arr[x], returnValue;
        if(arr[x]>0)   
            arr[x] = 0;//setting the actual element of the array to 0
        else
            arr[x]--;// decrementing the search index so it goes from 0-> -1 -> -2 -> -3...
        if(Math.abs(arr[-arr[x]]) == x)//We check if the number is at our search index...
            returnValue = 0;
        else        
            returnValue = findElement(arr, x)+1;
        arr[x] = currValue;//We take the value of the index from when the function was called and then reassign it to the same index after our work with it is done.

        return returnValue;
}

Since the array only has to be the same after execution and it doesn't matter it's state during execution, this may work. Note: I haven't done elaborate test on this so please do test the code sometimes before submitting

Answer 3

I have assumed arr can be modified, provide it is unchanged after the answer has been obtained.

Since it is only "preferable" that the answer be in Java (and I don't know Java), I'll offer a solution in Ruby. With its pseudo-code appearance and added comments readers unfamiliar with Ruby should be able to follow the calculations.

Specifically, I append an element to the given array which equals the index of the current element of the array to be examined (initially zero). If that element equals the target value we return up the recursion chain, initially returning zero, then adding one at each subsequent point of the chain. Before returning the desired count in doit , the last element of the array is removed to restore the array to its initial value.

If the value of the array indexed by the last element of the array (the current index) does not equal the target value the last element of the array is incremented by one and the method is called recursively.

def doit(arr,target)
  arr << 0 # append the index 0 to arr
  n = recurse(arr, target)
  arr.pop  # remove the last element of arr
  n
end

def recurse(arr, target)
  return 0 if arr[arr[-1]] == target
  arr[-1] += 1 # increment last value of arr by 1
  1 + recurse(arr, target)
end

arr = [4, 1, 0, 2, 3]

doit(arr, 4) #=> 0
doit(arr, 1) #=> 1
doit(arr, 0) #=> 2
doit(arr, 2) #=> 3
doit(arr, 3) #=> 4