Fixed some bugs.

Author: a1fredbao

src/alfred/data_structure/cartesian.hpp

@@ -1,7 +1,7 @@
 #ifndef AFDS_CARTESIAN
 #define AFDS_CARTESIAN
 
-#include <pair>
+#include <utility>
 #include <vector>
 
 // Returns pair of (ls, rs).

src/alfred/math/comb.hpp

@@ -33,6 +33,7 @@ class Comb {
         return _invfac[m];
     }
     inline mint inv(int m) {
+        if (m < 0) return -inv(-m);
         if (m > n) init(m);
         return _inv[m];
     }