大数加法

在进行加法是若符号不同则进行减法，减法带绝对值，不然最高位会出现负数，不便于输出

#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;

struct bign {
	int d[1000];
	int len, flag; //flag 标识正数和负数 1 为负数 0 正数 
	
	bign() {
		fill(d, d + 1000, 0);
		len = 0;
		flag = 0; //默认为正数
	}
};

bign change(char str[]) {
	bign a;
	if (str[0] == '-') a.flag = 1;
	
	a.len = strlen(str);
	for (int i = 0; i < a.len; i++) {
		if (str[a.len - i - 1] >= '0' && str[a.len - i - 1] <= '9') {
			a.d[i] = str[a.len - i - 1] - '0';
		}
	}
	
	if (str[0] == '+' || str[0] == '-') a.len--;
	
	return a;
}

//去除前导0，但保证有一位为0
void dislodge(bign &a) {
	while (a.len - 1 > 0 && a.d[a.len - 1] == 0) a.len--;
}

//a > b返回1 相等返回0 小于返回-1 
int compare(bign a, bign b) {
	if (a.len > b.len) return 1;
	else if (a.len < b.len) return -1;
	else {
		for (int i = a.len - 1; i >= 0; i--) {
			if (a.d[i] > b.d[i]) return 1;
			else if( a.d[i] < b.d[i]) return -1;
		}
	}
	
	return 0;
}

bign add(bign a, bign b) {
	bign c;
	
	int carray = 0;
	for (int i = 0; i < a.len || i < b.len; i++) {
		int temp = a.d[i] + b.d[i] + carray;
		c.d[c.len++] = temp % 10;
		carray = temp / 10;
	}
	
	if (carray != 0) c.d[c.len++] = carray;
	
	dislodge(c);
	
	return c;
}

//减法 (a - b)
bign sub(bign a, bign b) {
	bign c;
	
	for (int i = 0; i < a.len || i < b.len; i++) {
		if (a.d[i] < b.d[i] && (i + 1) < a.len) {
			a.d[i+1]--;
			a.d[i] += 10;
		}
		
		c.d[c.len++] = abs(a.d[i] - b.d[i]);
	}
	
	dislodge(c);
	
	return c;
} 
 

int main() {
	char stra[1001], strb[1001];
	
	scanf("%s %s", stra, strb);
	
	bign a = change(stra);
	bign b = change(strb);
	
	bign c;
	if (a.flag == b.flag) c = add(a, b);
	else c = sub(a, b);
	
	// 判断符号
	if (compare(a, b) == -1) c.flag = 1; 
	
	if (c.flag == 1) printf("-"); 
	for (int i = c.len - 1; i >= 0; i--) {
		printf("%d", c.d[i]);
	}

	return 0;
}