Find last index- Recursion challenge

#1

Can you please explain the use of this statement properly like a dry run as i’m not able to get when the recursive calls are happening exactly.
"int rr= lastIndex( arr, n, key, idx+1); "

#include

using namespace std;
int lastIndex( int arr[], int n,int key, int idx)
{
//base case
if(idx == n)
return -1;
int rr= lastIndex( arr, n, key, idx+1);

//rec case
if(key== arr[idx] && rr == (-1))
{
    return idx;
}
  return rr;

}
int main()
{
int n;
cin>>n;
int a[1000];
for( int i=0; i<n; i++)
cin>>a[i];
int key;
cin>>key;
cout<<lastIndex(a, n, key, 0);
return 0;
}

#2

@samk86676, the trick to understand recursion is assuming the function will do its work all we need to do is utilize the result , if it does’nt make any sense then let me explain, so here , lastIndex(arr,n,ket,idx) function will give you the last index of key if present in the range idx to n-1 so what we are doing is for every index we are calling in the range[index+1,n) if the last index is present in this range then we will simply return the value returned by the function as even if arr[index] is same as key it won’t be the last Index as there is already a key on the right, if the value returned is -1 this means the key is not present in the range[index+1,n) so now we will check if arr[index] is key or not if yes then we will return the index if no then we will return -1 ,

#3

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