1 大数：
2 #include <stdio.h>
3 #include <string.h>
4 #include <stdlib.h>
5 #include <math.h>
6 #include <assert.h>
7 #include <ctype.h>
8 #include <map>
9 #include <string>
10 #include <set>
11 #include <bitset>
12 #include <utility>
13 #include <algorithm>
14 #include <vector>
15 #include <stack>
16 #include <queue>
17 #include <iostream>
18 #include <fstream>
19 #include <list>
20 using namespace std;
21
22 const int MAXL = 500;
23 struct BigNum
24 {
25 int num[MAXL];
26 int len;
27 };
28
29 //高精度比较 a > b return 1, a == b return 0; a < b return -1;
30 int Comp(BigNum &a, BigNum &b)
31 {
32 int i;
33 if(a.len != b.len) return (a.len > b.len) ? 1 : -1;
34 for(i = a.len-1; i >= 0; i--)
35 if(a.num[i] != b.num[i]) return (a.num[i] > b.num[i]) ? 1 : -1;
36 return 0;
37 }
38
39 //高精度加法
40 BigNum Add(BigNum &a, BigNum &b)
41 {
42 BigNum c;
43 int i, len;
44 len = (a.len > b.len) ? a.len : b.len;
45 memset(c.num, 0, sizeof(c.num));
46 for(i = 0; i < len; i++)
47 {
48 c.num[i] += (a.num[i]+b.num[i]);
49 if(c.num[i] >= 10)
50 {
51 c.num[i+1]++;
52 c.num[i] -= 10;
53 }
54 }
55 if(c.num[len])
56 len++;
57 c.len = len;
58 return c;
59 }
60 //高精度减法,保证a >= b
61 BigNum Sub(BigNum &a, BigNum &b)
62 {
63 BigNum c;
64 int i, len;
65 len = (a.len > b.len) ? a.len : b.len;
66 memset(c.num, 0, sizeof(c.num));
67 for(i = 0; i < len; i++)
68 {
69 If(i>=b.len)
70 c.num[i]+=a.num[i];
71 else
72 c.num[i] += (a.num[i]-b.num[i]);
73 if(c.num[i] < 0)
74 {
75 c.num[i] += 10;
76 c.num[i+1]--;
77 }
78 }
79 while(c.num[len] == 0 && len > 1)
80 len--;
81 c.len = len;
82 return c;
83 }
84 //高精度乘以低精度，当b很大时可能会发生溢出int范围,具体情况具体分析
85 //如果b很大可以考虑把b看成高精度
86 BigNum Mul1(BigNum &a, int &b)
87 {
88 BigNum c;
89 int i, len;
90 len = a.len;
91 memset(c.num, 0, sizeof(c.num));
92 //乘以0，直接返回0
93 if(b == 0)
94 {
95 c.len = 1;
96 return c;
97 }
98 for(i = 0; i < len; i++)
99 {
100 c.num[i] += (a.num[i]*b);
101 if(c.num[i] >= 10)
102 {
103 c.num[i+1] = c.num[i]/10;
104 c.num[i] %= 10;
105 }
106 }
107 while(c.num[len] > 0)
108 {
109 c.num[len+1] = c.num[len]/10;
110 c.num[len++] %= 10;
111 }
112 c.len = len;
113 return c;
114 }
115
116 //高精度乘以高精度,注意要及时进位，否则肯能会引起溢出,但这样会增加算法的复杂度，
117 //如果确定不会发生溢出, 可以将里面的while改成if
118 BigNum Mul2(BigNum &a, BigNum &b)
119 {
120 int i, j, len = 0;
121 BigNum c;
122 memset(c.num, 0, sizeof(c.num));
123 for(i = 0; i < a.len; i++)
124 {
125 for(j = 0; j < b.len; j++)
126 {
127 c.num[i+j] += (a.num[i]*b.num[j]);
128 if(c.num[i+j] >= 10)
129 {
130 c.num[i+j+1] += c.num[i+j]/10;
131 c.num[i+j] %= 10;
132 }
133 }
134 }
135 len = a.len+b.len-1;
136 if(c.num[len])
137 len++;
138 while(c.num[len-1] == 0 && len > 1)
139 len--;
140 c.len = len;
141 return c;
142 }
143
144 //高精度除以低精度,除的结果为c, 余数为f
145 void Div1(BigNum &a, int &b, BigNum &c, int &f)
146 {
147 int i, len = a.len;
148 memset(c.num, 0, sizeof(c.num));
149 f = 0;
150 for(i = a.len-1; i >= 0; i--)
151 {
152 f = f*10+a.num[i];
153 c.num[i] = f/b;
154 f %= b;
155 }
156 while(len > 1 && c.num[len-1] == 0)
157 len--;
158 c.len = len;
159 }
160 //高精度*10
161 void Mul10(BigNum &a)
162 {
163 int i, len = a.len;
164 for(i = len; i >= 1; i--)
165 a.num[i] = a.num[i-1];
166 a.num[i] = 0;
167 len++;
168 //if a == 0
169 while(len > 1 && a.num[len-1] == 0)
170 len--;
171 a.len=len;
172 }
173
174 //高精度除以高精度，除的结果为c，余数为f
175 void Div2(BigNum &a, BigNum &b, BigNum &c, BigNum &f)
176 {
177 int i, len = a.len;
178 memset(c.num, 0, sizeof(c.num));
179 memset(f.num, 0, sizeof(f.num));
180 f.len = 1;
181 for(i = len-1;i >= 0;i--)
182 {
183 Mul10(f);
184 //余数每次乘10
185 f.num[0] = a.num[i];
186 //然后余数加上下一位
187 ///利用减法替换除法
188 while(Comp(f, b) >= 0)
189 {
190 f = Sub(f, b);
191 c.num[i]++;
192 }
193 }
194 while(len > 1 && c.num[len-1] == 0)
195 len--;
196 c.len = len;
197 }
198 void print(BigNum &a) //输出大数
199 {
200 int i;
201 for(i = a.len-1; i >= 0; i--)
202 printf("%d", a.num[i]);
203 puts("");
204 }
205 //将字符串转为大数存在BigNum结构体里面
206 BigNum ToNum(char *s)
207 {
208 int i, j;
209 BigNum a;
210 a.len = strlen(s);
211 for(i = 0, j = a.len-1; s[i] != '\0'; i++, j--)
212 a.num[i] = s[j]-'0';
213 return a;
214 }
215
216 void Init(BigNum &a, char *s, int &tag) //将字符串转化为大数
217 {
218 int i = 0, j = strlen(s);
219 if(s[0] == '-')
220 {
221 j--;
222 i++;
223 tag *= -1;
224 }
225 a.len = j;
226 for(; s[i] != '\0'; i++, j--)
227 a.num[j-1] = s[i]-'0';
228 }
229
230 int main(void)
231 {
232 BigNum a, b;
233 char s1[100], s2[100];
234 while(scanf("%s %s", s1, s2) != EOF)
235 {
236 int tag = 1;
237 Init(a, s1, tag); //将字符串转化为大数
238 Init(b, s2, tag);
239 a = Mul2(a, b);
240 if(a.len == 1 && a.num[0] == 0)
241 {
242 puts("0");
243 }
244 else
245 {
246 if(tag < 0) putchar('-');
247 print(a);
248 }
249 }
250 return 0;
251 }
