大整数运算

#include <cstdlib>
#include <cstdio>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

void reverse(char*s) {
	if (s == NULL ) return;
	char *e = s + strlen(s) - 1;
	while (s < e) swap(*s, *e), s++, e--;
}
char* add(char* a, char* b) {
	if (a == NULL || b == NULL) return NULL;

	int la = strlen(a), lb = strlen(b);
	int lc = max(la, lb) + 2;
	char*c = new char[lc];
	c[0] = '\0';
	reverse(a);
	reverse(b);

	int carr = 0;
	int i = 0, t;
	while (i < la && i < lb) {
		t = a[i] - '0' + b[i] - '0' + carr;
		carr = t / 10;
		c[i] = (t % 10) + '0';
		i++;
	}
	while (i < la) {
		t = a[i] - '0' + carr;
		carr = t / 10;
		c[i++] = (t % 10) + '0';
	}
	while (i < lb) {
		t = b[i] - '0' + carr;
		carr = t / 10;
		c[i++] = (t % 10) + '0';
	}
	if (carr != 0) c[i++] = '1';
	c[i] = '\0';
	reverse(a);
	reverse(b);
	reverse(c);
	return c;
}

int cmp(char *a, char *b) {
	int la = strlen(a), lb = strlen(b);
	if (la != lb) return la - lb;
	else {
		while (*a != '\0' && *a == *b) a++, b++;
		return *a - *b;
	}
}
char* minus(char *a, char *b) {
	if (a == NULL || b == NULL) return NULL;
	if (cmp(a, b) < 0) {
		char *c = minus(b, a);
		reverse(c);
		int l = strlen(c);
		c[l] = '-';
		c[l+1] = '\0';
		reverse(c);
		return c;
	}
	int la = strlen(a), lb = strlen(b);
	int lc = max(la, lb) + 2;
	char*c = new char[lc];
	c[0] = '\0';
	reverse(a);
	reverse(b);

	int carr = 0;
	int i = 0, t;
	while (i < la && i < lb) {
		t = a[i] - b[i] + carr;
		if (t < 0) carr = -1, t += 10;
		else carr = 0;
		c[i++] = t + '0';
	}
	while (i < la) {
		t = a[i] + carr - '0';
		if (t < 0) carr = -1, t += 10;
		else carr = 0;
		c[i++] = t + '0';
	}
	while (--i > 0 && c[i] == '0') ;
	c[i+1] = '\0';
	reverse(a);
	reverse(b);
	reverse(c);
	return c;
}

int main() {
	char a[] = "2223";
	char b[] = "222999";
	auto res = minus(a, b);
	return 0;
}