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整数の多桁積なら[ココ](http://codezine.jp/article/detail/8813)に僕が書いたのがあるから、こいつを6回呼びなされ。
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[追記] やってみた。
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```C++
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#include <vector>
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#include <string>
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#include <algorithm>
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using namespace std;
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string onpaper_multiplies(const string& sa, const string& sb) {
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int digits = (int)(sa.size() + sb.size());
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vector<int> a(sa.size(), 0);
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vector<int> b(sb.size(), 0);
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vector<int> c(digits, 0);
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// 下位の桁から順にセット
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auto char2int = [](char ch) { return ch - '0'; };
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transform(sa.rbegin(), sa.rend(), a.begin(), char2int);
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transform(sb.rbegin(), sb.rend(), b.begin(), char2int);
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// a,b各桁の全組み合わせで掛け算を行う
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for (size_t i = 0; i < a.size(); ++i) {
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for (size_t j = 0; j < b.size(); ++j) {
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c[i + j] += a[i] * b[j];
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}
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}
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// 桁あふれの繰り上げ
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int carry = 0;
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transform(c.begin(), c.end(), c.begin(),
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[&](int n) { int t = n + carry; carry = t / 10; return t % 10; });
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// 文字列化（ゼロ・サプレスを行う）
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string result;
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result.reserve(digits);
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bool zero = false;
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for_each(c.rbegin(), c.rend(),
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[&](int ch) { if (ch != 0 && !zero) { zero = true; }
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if (zero) { result.push_back(ch + '0'); } });
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return result;
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}
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#define TRIAL
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#ifdef TRIAL
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#include <iostream>
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using namespace std;
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int main() {
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string sa = "1";
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string sb = "87654321";
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string sc;
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for ( int i = 0; i < 6; ++i ) {
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sc = onpaper_multiplies(sa, sb);
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sa = sc;
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}
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cout << sc << endl;
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}
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#endif
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```
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コタエは 453565464948711215734134218664154378043866821921 だそうです。
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