1049 数列的片段和 （20 分）

需要注意的一点：

当B的长度小于A的长度时，需要的B的前面补零。

#include <iostream>
#include<string>
#include<algorithm>
using namespace std;
int main()
{
	string str1, str2;
	cin >> str1 >> str2;
	int len1 = str1.length();
	int len2 = str2.length();
	char str1_f[105], str2_f[105];
	int str3_f[105];
	//将两个字符串倒置
	//reverse(begin,end)会将区间[begin,end)内的元素全部逆序
	//reverse(str1.begin().str1.end());
	for (int i = 1;i <= len1;i++) {
		str1_f[i] = str1[len1 - i];
	}
	for (int i = 1;i <= len2;i++) {
		str2_f[i] = str2[len2 - i];
	}
	//B的长度小于等于A的长度时，
	if (len2 <= len1) {
		for (int i = 1;i <= len2;i++) {
			if (i % 2 == 0) {//偶数位时
				int b = str2_f[i] - str1_f[i];
				if (b < 0) {
					b += 10;
				}
				str3_f[i] = b;
			}
			else {//奇数位时
				int b = str2_f[i] - '0' + str1_f[i] - '0';
				b = b % 13;
				str3_f[i] = b;
			}
		}
		//需要在B的前面补零
		for (int j = len2 + 1;j <= len1;j++) {
			if (j % 2 == 0) {//偶数位时
				int b = '0' - str1_f[j];
				if (b < 0) {
					b += 10;
				}
				str3_f[j] = b;
			}
			else {//奇数位时
				int b = str1_f[j] - '0';
				str3_f[j] = b;
			}
		}
		for (int j = len1;j > 0;j--) {
			if (str3_f[j] < 10) {
				cout << str3_f[j];
			}
			else {
				if (str3_f[j] == 10) {
					cout << 'J';
				}
				if (str3_f[j] == 11) {
					cout << 'Q';
				}
				if (str3_f[j] == 12) {
					cout << 'K';
				}
			}
		}

	}
	//B的长度大于A的长度时
	else {//len2大于len1时
		for (int i = 1;i <= len1;i++) {
			if (i % 2 == 0) {//偶数位时
				int b = str2_f[i] - str1_f[i];
				if (b < 0) {
					b += 10;
				}
				str3_f[i] = b;
			}
			else {//奇数位时
				int b = str2_f[i] - '0' + str1_f[i] - '0';
				b = b % 13;
				str3_f[i] = b;
			}
		}
		for (int i = len1 + 1;i <= len2;i++) {
			str3_f[i] = str2_f[i]-'0';
		}
		for (int j = len2;j > 0;j--) {
			if (str3_f[j] < 10) {
				cout << str3_f[j];
			}
			else {
				if (str3_f[j] == 10) {
					cout << 'J';
				}
				if (str3_f[j] == 11) {
					cout << 'Q';
				}
				if (str3_f[j] == 12) {
					cout << 'K';
				}
			}
		}
	}
	
	return 0;
}