chore: ensure any class can initialize with no parameters

Author: a1fredbao

src/alfred/data_structure/dsu/cancel-dsu.hpp

@@ -9,9 +9,14 @@ class CancelDSU {
     std::vector<int> fa, siz, S;
 
 public:
+    CancelDSU(void) = default;
     CancelDSU(int n) : fa(n + 1), siz(n + 1, 1) {
         std::iota(fa.begin(), fa.end(), 0);
     }
+    inline void init(int n) {
+        n++, fa.resize(n), siz.assign(n, 1);
+        std::iota(fa.begin(), fa.end(), 0);
+    }
     inline int find(int x) {
         return fa[x] == x ? x : find(fa[x]);
     }

src/alfred/data_structure/dsu/dsu.hpp

@@ -6,9 +6,14 @@
 
 struct DSU {
     std::vector<int> fa, siz;
+    DSU(void) = default;
     DSU(int n) : fa(n + 1), siz(n + 1, 1) {
         std::iota(fa.begin(), fa.end(), 0);
     }
+    inline void init(int n) {
+        n++, fa.resize(n), siz.assign(n, 1);
+        std::iota(fa.begin(), fa.end(), 0);
+    }
     inline int find(int x) {
         return fa[x] == x ? x : fa[x] = find(fa[x]);
     }

src/alfred/data_structure/dsu/weighted-dsu.hpp

@@ -8,9 +8,14 @@ template <class T>
 struct WeightedDSU {
     std::vector<int> fa;
     std::vector<T> w;
+    WeightedDSU(void) = default;
     WeightedDSU(int n) : fa(n + 1), w(n + 1) {
         std::iota(fa.begin(), fa.end(), 0);
     }
+    inline void init(int n) {
+        n++, fa.resize(n), w.resize(n);
+        std::iota(fa.begin(), fa.end(), 0);
+    }
     inline int find(int x) {
         if (fa[x] == x) return x;
         int f = fa[x], f2 = find(f);

src/alfred/data_structure/fenwick.hpp

@@ -5,23 +5,27 @@
 
 template <class T>
 struct Fenwick {
+    int n;
     std::vector<T> c;
+    Fenwick(void) = default;
+    Fenwick(int len) : n(len + 2), c(len + 2) {}
+    inline void init(int _n) { n = _n + 2, c.resize(n); }
     inline int lowbit(int x) { return x & -x; }
     inline void merge(T &x, T &y) { x = x + y; }
     inline T subtract(T x, T y) { return x - y; }
-    inline void update(size_t pos, T x) {
-        for (pos++; pos < c.size(); pos += lowbit(pos)) merge(c[pos], x);
+    inline void update(int pos, T x) {
+        for (pos++; pos < n; pos += lowbit(pos)) merge(c[pos], x);
     }
     inline void clear(void) {
         for (auto &x : c) x = T();
     }
-    inline T query(size_t pos) {
+    inline T query(int pos) {
         T ans = T();
         for (pos++; pos; pos ^= lowbit(pos)) merge(ans, c[pos]);
         return ans;
     }
-    inline T query(size_t l, size_t r) {
-        return l == 0 ? query(r) : subtract(query(r), query(l - 1));
+    inline T query(int l, int r) {
+        return subtract(query(r), query(l - 1));
     }
     inline int kth(T k) {
         int ans = 0;
@@ -32,7 +36,6 @@ struct Fenwick {
         }
         return ans;
     }
-    Fenwick(size_t len) : c(len + 2) {}
 };
 
 #endif // AFDS_FENWICK