Improve inv and bisect

Author: adamant-pwn

cp-algo/math/poly.hpp

@@ -546,15 +546,16 @@ namespace cp_algo::math {
         }
 
         // Return {P0, P1}, where P(x) = P0(x) + xP1(x)
-        std::array<poly_t, 2> bisect() const {
-            std::vector<T> res[2];
-            res[0].reserve(deg() / 2 + 1);
-            res[1].reserve(deg() / 2 + 1);
-            for(int i = 0; i <= deg(); i++) {
+        std::array<poly_t, 2> bisect(size_t n) const {
+            std::deque<T> res[2];
+            for(size_t i = 0; i < n; i++) {
                 res[i % 2].push_back(a[i]);
             }
             return {res[0], res[1]};
         }
+        std::array<poly_t, 2> bisect() const {
+            return bisect(size(a));
+        }
 
         // Find [x^k] P / Q
         static T kth_rec(poly_t P, poly_t Q, int64_t k) {

cp-algo/math/poly/impl/div.hpp

@@ -91,12 +91,12 @@ namespace cp_algo::math::poly::impl {
     }
     template<typename poly>
     poly inv(poly const& p, size_t n) {
-        auto q = p.mod_xk(n);
         if(n == 1) {
-            return poly(1) / q[0];
+            return poly(1) / p[0];
         }
         // Q(-x) = P0(x^2) + xP1(x^2)
-        auto [q0, q1] = q.negx().bisect();
+        auto [q0, q1] = p.bisect(n);
+        q1 *= -1;
 
         int N = fft::com_size((n + 1) / 2, (n + 1) / 2);