大整数乘法

大整数乘法

http://blog.sina.com.cn/s/blog_7496d1d601010o4e.html
原博地址

#include<iostream>using namespace std;#define null 0#define MAXN 51//大数相乘char *big_cheng(char line1[], char line2[]){short s1[MAXN], s2[MAXN], s[MAXN];//s1，s2：两个乘数；s：乘积的结果int i, j, k, c;int len1, len2, len;len1 = strlen(line1);len2 = strlen(line2);len = len1 + len2;//初步确定乘积的长度for(i = len1 - 1; i >= 0; i--){s1[i] = line1[len1 - i -1] - '0';//将字符串转化为整型，并反着存放}for(i = len2 - 1; i >= 0; i--){s2[i] = line2[len2 - i - 1] - '0'; }memset(s, 0, sizeof(short) * (MAXN - 1));//s数组初始化for(i = 0; i <= len1 -1; i++)for(j = 0; j <= len2 -1; j++)s[i + j] = s[i + j] + s1[i] * s2[j];//逐位相乘for(i = 0, c = 0; i <= len - 1;i++)//处理进位{k = s[i] + c;s[i] = k % 10;c = k / 10;}for(i = len - 1; i >= 0; i--)if(s[i] != 0)break;//处理多余的零len = i + 1;char *line;//注意只有指针才可以返回去，不能定义成数组line = new char[len + 1];if(len == 0){line[0] = 0 + '0';line[1] = '\0';//字符串的结束标志}else{for(i = 0; i <= len - 1; i++)line[i] = s[len - i - 1] + '0';line[len] = '\0';//字符串的结束标志 } return line;}int main(){char line1[MAXN], line2[MAXN];while(cin >> line1 >>line2){cout << big_cheng(line1, line2) << endl;}return 0;}