#include <IOSTREAM>
using namespace std;

long long f_AtoI(char str[]) // 字符串转为整数,通过减'0'字符,底层用ASCII码相减
{
	int i = 0;
	long long tempI = 0;

	while (str[i] != '\0')
	{
		tempI = tempI * 10 + (str[i] - '0'); // 与字符0的差距
		// cout << "i=" << i << " Get=" << (str[i] - '0') << endl;
		++i;
	}

	return tempI; // 转换后赋值给m
}

char *f_ItoA(long long n)
{
	static char tmpStr[100]{0};
	static char str[]{0};

	int i(0);
	int j = 0;

	while (n)
	{
		tmpStr[i++] = n % 10 + '0';
		n /= 10;
	}
	// 此时为逆序，需要调整为正序

	// cout << "tmpStr=" << tmpStr << endl;
	while (i > 0)
		str[j++] = tmpStr[--i];
	// cout << "str=" << str << endl;
	return str;
}

int main()
{
	char str[] = "5000000050000000";
	long long llngN = 0;
	
	llngN = f_AtoI(str);
	cout << "llngN=" << llngN << endl;

	string str2 = f_ItoA(llngN);
	cout << "str2=" << str2 << endl;

	system("pause");
	return 0;
}

运行结果：

llngN=5000000050000000
str2=5000000050000000
请按任意键继续. . .