792. 高精度减法

#include <iostream>
#include <vector>
using namespace std;

bool cmp(vector<int> A, vector<int> B)
{
    if (A.size() != B.size())    return A.size() > B.size();
    
    for (int i = A.size() - 1; i >= 0; -- i)
        if (A[i] != B[i])    return A[i] > B[i];
    
    return true;
}

vector<int> sub(vector<int> A, vector<int> B)
{
    vector<int> C;
    int t = 0;
    
    for (int i = 0; i < A.size(); ++ i)
    {
        t = A[i] - t;
        if (i < B.size())   t -= B[i];
        C.push_back((t + 10) % 10);
        if (t < 0)  t = 1;
        else    t = 0;
    }
    
    // 此处C.size() > 1 而不是 > 0 是输出结果为0的正确答案
    while (C.size() > 1 && C.back() == 0)   C.pop_back();
    
    return C;
}

int main()
{
    string a, b;
    cin >> a >> b;
    
    vector<int> A, B, C;
    for (int i = a.size() - 1; i >= 0; -- i)    A.push_back(a[i] - '0');
    for (int i = b.size() - 1; i >= 0; -- i)    B.push_back(b[i] - '0');
    
    if (cmp(A, B))  C = sub(A, B);
    else
    {
        C = sub(B, A);
        cout << "-";
    }
    
    for (int i = C.size() - 1; i >= 0; -- i)    cout << C[i];
    
    return 0;
}