diff --git a/trees/BIT.rs b/trees/BIT.rs
new file mode 100644
index 0000000..dbb5c4b
--- /dev/null
+++ b/trees/BIT.rs
@@ -0,0 +1,23 @@
+struct BIT(usize, Vec<i64>);
+
+impl BIT {
+    fn new(n: usize) -> Self {
+        Self(n, vec![0;n+1])
+    }
+    fn query(&self, mut i: usize) -> i64 {
+        let lowbit = |x: i64|x&-x;
+        let mut res = 0;
+        while i > 0 {
+            res += self.1[i];
+            i -= lowbit(i as i64) as usize;
+        }
+        res
+    }
+    fn insert(&mut self, mut i: usize) {
+        let lowbit = |x: i64| x & -x;
+        while i <= self.0 {
+            self.1[i] += 1;
+            i += lowbit(i as i64) as usize;
+        }
+    }
+}