Add tourist fft

Author: samuraiexx

tests/math/fft-tourist.test.cpp

@@ -0,0 +1,51 @@
+// @problem_url: https://www.spoj.com/problems/MUL/
+#include<bits/stdc++.h>
+
+using namespace std;
+typedef long double ld;
+
+// @include: math/fft-tourist.cpp
+
+int main() {
+  int n;
+  cin >> n;
+
+  while (n--) {
+    string a, b;
+    cin >> a >> b;
+
+    int size = a.size() + b.size();
+    while (size - (size & -size) != 0) size -= size & -size;
+    size *= 2;
+
+    vector<int> va(size);
+    vector<int> vb(size);
+
+    for (int i = 0; i < a.size(); i++) va[a.size() - i - 1] = a[i] - '0';
+    for (int i = 0; i < b.size(); i++) vb[b.size() - i - 1] = b[i] - '0';
+
+    auto ans = fft::multiply(va, vb);
+
+    string answer;
+    int goes = 0;
+
+    for (auto c : ans) {
+      int x = c + goes;
+
+      goes = 0;
+      if (x > 9) {
+        goes = x / 10;
+        x = x % 10;
+      }
+
+      answer.push_back(x + '0');
+    }
+    answer.push_back(goes + '0');
+
+    while (answer.back() == '0') answer.pop_back();
+    if (answer.empty()) answer.push_back('0');
+    reverse(answer.begin(), answer.end());
+
+    cout << answer << endl;
+  }
+}