ft_itoa

주의점

int에서 char로 갈 때 반드시 ‘0’을 더해준다.
c
```
int a;
char b;

b = a + '0'
```
음수 처리
0 처리
int 최소 음수 처리

문제

Assignment name  : ft_itoa
Expected files   : ft_itoa.c
Allowed functions: malloc
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Write a function that takes an int and converts it to a null-terminated string.
The function returns the result in a char array that you must allocate.

Your function must be declared as follows:

char	*ft_itoa(int nbr);