C 终端输入 字符123 输出 10进制123

#include <stdio.h>

#define N 20
int main(int argc, const char *argv[])
{
	char a[N] = {'\0'};
	int i = 0;
	char ch = '\0';
	while((ch=getchar())!='\n')
	{
		a[i++] = ch;
	}
	int cnt = i;
	int num  = 0;
	for(i=0;i<cnt;i++)
	{
		num = num * 10 + (a[i] - '0');
	}
////////////////////////////////////
        printf("num = %d\n",num);
	return 0;
}