diff --git a/segtree/segtree-d.cc b/segtree/segtree-d.cc
new file mode 100644
index 0000000..42ec878
--- /dev/null
+++ b/segtree/segtree-d.cc
@@ -0,0 +1,24 @@
+constexpr int MAXN=1e5+10;
+int n,cnt,root;
+int d[MAXN*2],ls[MAXN*2],rs[MAXN*2];
+
+void update(int s,int t,int &p,int x,int c){  // add constant c to #x
+    if(!p)p=++cnt;  // create node
+    if(s==t){
+        d[p]+=c;
+        return;
+    }
+    int m=s+(t-s>>1);
+    if(x<=m)update(ls[p],s,m,x,c);
+    else    update(rs[p],m+1,t,x,c);
+    d[p]=d[ls[p]]+d[rs[p]];
+}
+
+int query(int s,int t,int p,int l,int r){  // query range [l,r]
+    if(!p)return 0;
+    if(l<=s&&t<=r)return d[p];
+    int m=s+(t-s>>1),sum=0;
+    if(l<=m)sum+=query(ls[p],s,m,l,r);
+    if(r>m) sum+=query(rs[p],m+1,t,l,r);
+    return sum;
+}
diff --git a/segtree/segtree.cc b/segtree/segtree.cc
new file mode 100644
index 0000000..d7d09fd
--- /dev/null
+++ b/segtree/segtree.cc
@@ -0,0 +1,42 @@
+constexpr int MAXN=1e5+10;
+int a[MAXN],d[4*MAXN],b[4*MAXN];  // tree d, tags b
+
+void build(int s,int t,int p){  // root p, range [s,t]
+    if(s==t){
+        d[p]=a[s];
+        return;
+    }
+    int m=s+(t-s>>1);
+    build(s,m,p*2),build(m+1,t,p*2+1);
+    d[p]=d[p*2]+d[p*2+1];
+}
+
+int getsum(int s,int t,int p,int l,int r){
+    if(l<=s&&t<=r)return d[p];
+    int m=s+(t-s>>1),sum=0;
+    if(b[p]){
+        d[p*2]+=b[p]*(m-s+1),d[p*2+1]+=b[p]*(t-m);
+        b[p*2]+=b[p],b[p*2+1]+=b[p];
+        b[p]=0;
+    }
+    if(l<=m)sum+=getsum(l,r,s,m,p*2);
+    if(r>m) sum+=getsum(l,r,m+1,t,p*2+1);
+    return sum;
+}
+
+void update(int s,int t,int p,int l,int r,int c) {  // difference c
+    // replace `+=` with `=` to implement modifications
+    if(l<=s&&t<=r){
+        d[p]+=(t-s+1)*c,b[p]+=c;
+        return;
+    }
+    int m=s+(t-s>>1);
+    if(b[p]&&s!=t){
+        d[p*2]+=b[p]*(m-s+1),d[p*2+1]+=b[p]*(t-m);
+        b[p*2]+=b[p],b[p*2+1]+=b[p];
+        b[p]=0;
+    }
+    if(l<=m)update(l,r,c,s,m,p*2);
+    if(r>m) update(l,r,c,m+1,t,p*2+1);
+    d[p]=d[p*2]+d[p*2+1];
+}