[求助]C语言实现类型转换的问题!

怎么样把一个数字转换成字符?
就像把:字符常量转换成数字一样:
int a;
char b;
a=atoi(b);

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我也是新手，胡乱写下，很多就不考虑了。你先看看：
/*
* itos.c:
* convert decimal integer to equivalent string.
*/

#include <stdio.h>
#define MAX 20

char *itos(long long int i, char s[])
{
long long int n = 0;
unsigned ind;
char snum[] = "0123456789";

for (; i > 10; i /= 10)
n = (n + i % 10) * 10;
n += i;

for (ind = 0; n != 0; ind++, n /= 10)
s[ind] = snum[n%10];

s[ind] = '\0';

return s;
}

int main(void)
{
long long int i1 = 123456789;
char s[MAX];

printf("test...\n%s\n", itos(i1,s));

return 0;
}

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