高精度加法函数

string a, b, jans;
int j; // 记得重置j,jans
int tmp;

string aandb() {
	stack<int> ans;
	if (a.length() < b.length()) swap(a, b);	
	for (int d1=a.length()-1;d1>=0;d1--) {
		int b_index = d1 - (a.length() - b.length());
        if (b_index >= 0 && b_index < b.length()) {
            tmp = b[b_index] - '0';
        } else {
            tmp = 0;
        }
		int w = (a[d1] - '0' ) + tmp + j;
		j = w / 10;
		ans.push(w % 10);
	}
	if (j != 0) ans.push(j);
	while (!ans.empty()) {
		jans +=  ans.top() + '0';
		ans.pop();
	}
	return jans;
}

P1601: https://www.luogu.com.cn/problem/P1601