add Fenwick::set/get

NASU41 · NASU41 · commit bc8a574a7183 · 2020-12-19T23:00:02.000+09:00
add new functions: set(int p, long x) and get(int p). I implemented this because I needed them on ABC186.
diff --git a/FenwickTree/FenwickTree.java b/FenwickTree/FenwickTree.java
@@ -16,6 +16,10 @@ public FenwickTree(long[] data) {
         build(data);
     }
 
+    public void set(int p, long x){
+        add(p, x - get(p));
+    }
+
     public void add(int p, long x){
         assert(0<=p && p<_n);
         p++;
@@ -29,6 +33,10 @@ public long sum(int l, int r){
         return sum(r)-sum(l);
     }
 
+    public long get(int p){
+        return sum(p, p+1);
+    }
+
     private long sum(int r){
         long s = 0;
         while(r>0){
diff --git a/FenwickTree/README.md b/FenwickTree/README.md
@@ -32,6 +32,19 @@ public void add(int p, long x)
 配列の第$p$要素に$x$を加える. すなわち, `a[p] += x` のこと.
 計算量: $O(\log N)$
 
+### set
+```java
+public void set(int p, long x)
+```
+配列の第$p$要素を$x$に変更する. すなわち, `a[p] = x` のこと.
+計算量: $O(\log N)$
+
+### get
+```java
+public long get(int p)
+```
+配列の第$p$要素を取得する. 計算量: $O(\log n)$
+
 ### sum
 ```java
 public long sum(int l, int r)
