模擬運算

用數組逆向保存大數每一位 加法直接從尾向前相加（尾部已對齊） 註意進位

減法類似 確保大數減小數 不夠減了向前減一 需要註意符號的有無問題

  1 #include <iostream>
  2 #include <cstring>
  3 #include <string>
  4 using namespace std;
  5 
  6 int a[11000], b[11000];
  7 string s1, s2;
  8 
  9 void Add()
 10 {
 11     int len1 = s1.length();
 12     int
 
 len2 = s2.length();
 13     int Max = max(len1, len2);
 14     int k;
 15     for (int i = len1-1, k = 0; i >= 0; i--)
 16         a[k++] = s1[i] - ‘0‘;
 17     for (int i = len2-1, k = 0; i >= 0; i--)
 18         b[k++] = s2[i] - ‘0‘;
 19 
 20     if (a[0] == 0 && b[0] == 0) {
 21         cout << ‘
 
0‘;
 22         return;
 23     }
 24     int up = 0;
 25     for (int i = 0; i < Max+1; i++) {
 26         a[i] = a[i] + b[i] + up;
 27         up = a[i] / 10;
 28         a[i] %= 10;
 29     }
 30     int first;
 31     for (first = Max+1; first >= 0; first--)
 32         if (a[first])
 33             break
 
;
 34 
 35     for (int i = first; i >= 0; i--)
 36         cout << a[i];
 37 
 38 }
 39 
 40 void Sub(string Max, string Min)  //a - b
 41 {
 42     int len1 = Max.length();
 43     int len2 = Min.length();
 44     int MaxLen = max(len1, len2);
 45 
 46     int k;
 47     for (int i = len1-1, k = 0; i >= 0; i--)
 48         a[k++] = Max[i] - ‘0‘;
 49     for (int i = len2 - 1, k = 0; i >= 0; i--)
 50         b[k++] = Min[i] - ‘0‘;
 51 
 52     int up = 0;
 53     for (int i = 0; i < MaxLen+1; i++) {
 54         a[i] = a[i] - b[i] - up;
 55         if (a[i] < 0) {
 56             up = 1;
 57             a[i] += 10;
 58         }
 59         else
 60             up = 0;
 61     }
 62 
 63     int first;
 64     for (first = MaxLen+1; first >= 0; first--)
 65         if (a[first])
 66             break;
 67 
 68     if (first == -1)
 69         cout << "0";
 70     else {
 71         for (int i = first; i >= 0; i--)
 72             cout << a[i];
 73     }
 74 }
 75 
 76 int main()
 77 {
 78     cin >> s1 >> s2;
 79 
 80     memset(a, 0, sizeof(a));
 81     memset(b, 0, sizeof(b));
 82 
 83     if (s1[0] == ‘-‘ && s2[0] == ‘-‘)
 84     {
 85         cout << ‘-‘;
 86         s1.erase(0, 1);
 87         s2.erase(0, 1);
 88         Add();
 89     }
 90     else if (s1[0] == ‘-‘ || s2[0] == ‘-‘)
 91     {
 92         string Max, Min;
 93         if (s1[0] == ‘-‘) {
 94             if (s1.length()-1 > s2.length())
 95                 cout << "-";
 96             if (s1.length()-1 == s2.length())
 97             {
 98                 if (s1[1] > s2[0])
 99                     cout << "-";
100             }
101             s1.erase(0, 1);
102         }
103 
104        if (s2[0] == ‘-‘) {
105 
106             if (s2.length()-1 > s1.length())
107                 cout << "-";
108             if (s1.length() == s2.length()-1)
109             {
110                 if (s2[1] > s1[0])
111                     cout << "-";
112             }
113             s2.erase(0, 1);
114         }
115 
116         if(s1.length() > s2.length())
117         {
118             Max = s1;
119             Min = s2;
120         }
121         else if (s1.length() == s2.length() && s1[0] > s2[0])
122         {
123             Max = s1;
124             Min = s2;
125         }
126         else {
127             Min = s1;
128             Max = s2;
129         }
130       Sub(Max, Min);
131     }
132     else
133         Add();
134 
135     return 0;
136 }

大整數加法與減法