Reversing a string but word wise! using char array. No string library.

#include <iostream>
#include<cstring>
using namespace std;

void reverseStringWordWise(char input[]) {
    // Write your code here
    int sp = strlen(input);
    int o_size = sp;
     char output[sp];
    int p = sp;
    int j = sp-1;
    int i=0;
    while(j>=0 || j==-1) // Since the first word will not have any space before it So it wont count
    {

        if(input[j]==' ' ||j==-1)
        {
            for(int k = j+1; k<p;k++) // j gets iterated from -1 to 0. Thus giving the frist word upto the the last space traced.
            {
                output[i++] = input[k];
            }
             p = j;
             j--;

            output[i++] = ' ';
        }
        else
        {
            j--;
        }

    }
  for(int l=0;l<sp;l++)
  {
      input[l] = output[l];
  }
}


int main() {
    char input[1000];
    cin.getline(input, 1000);
    reverseStringWordWise(input);
    cout << input << endl;
}