C/C++ :: Flip Char Array Input And Output It?

So my question was:

Write a program to read in a sequence of characters one by one. Print out the characters in reverse. You should use a char[]. (Remember single quotes are used for char)

For example:

Please enter characters one by one: (Enter 0 to exit)
h
e
l
l
o
0
You entered: hello.
The reverse of that is olleh.

and this is currently my code

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <ctime>
#include <cmath>
using namespace std;
int main() {
char entry[20];

[code]....

im just not sure how to set that value and still make the for loops work