NASU41 · NASU41 · Sep 9, 2020 · Sep 8, 2020 · Sep 8, 2020 · Sep 8, 2020
diff --git a/.gitignore b/.gitignore
@@ -24,4 +24,4 @@ hs_err_pid*
 
 # IntelliJ
 *.iml
-.idea/
+.idea/
diff --git a/FenwickTree/FenwickTree.java b/FenwickTree/FenwickTree.java
@@ -7,6 +7,15 @@ public FenwickTree(int n){
         data = new long[n];
     }
 
+    /**
+     * @verified https://atcoder.jp/contests/practice2/tasks/practice2_b
+     * @submission https://atcoder.jp/contests/practice2/submissions/16580495
+     */
+    public FenwickTree(long[] data) {
+        this(data.length);
+        build(data);
+    }
+
     public void add(int p, long x){
         assert(0<=p && p<_n);
         p++;
@@ -28,4 +37,14 @@ private long sum(int r){
         }
         return s;
     }
+
+    private void build(long[] dat) {
+        System.arraycopy(dat, 0, data, 0, _n);
+        for (int i=1; i<=_n; i++) {
+            int p = i+(i&-i);
+            if(p<=_n){
+                data[p-1] += data[i-1];
+            }
+        }
+    }
 }
diff --git a/FenwickTree/README.md b/FenwickTree/README.md
@@ -18,6 +18,13 @@ public FenwickTree(int n)
 長さ $n$ の配列 $a_0, a_1, \dots, a_{n-1}$を作ります. 初期値はすべて0です.
 計算量: $O(n)$
 
+```java
+public FenwickTree(long[] data)
+```
+
+長さ $n$ の配列 $a_0, a_1, \dots, a_{n-1}$を `data` により初期化します. 
+計算量: $O(n)$
+
 ## メソッド
 ### add
 ```java
-Original file line number
+Diff line change
@@ Expand Up / @@ -24,4 +24,4 @@ hs_err_pid* @@
     # IntelliJ
     *.iml
-    .idea/
+    .idea/