1 files changed, 51 insertions, 0 deletions
diff --git a/segmenttree.go b/segmenttree.go
new file mode 100644
index 0000000..e2be46a
--- /dev/null
+++ b/segmenttree.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"database/sql"
+)
+
+type segmentTree struct {
+	N   int
+	seg []int
+}
+
+// Build segment tree
+func (s segmentTree) build(a *sql.Rows, l, r, n int) {
+	if l == r {
+		a.Next()
+		a.Scan(&s.seg[n])
+		return
+	}
+	m := (l + r) >> 1
+	s.build(a, l, m, n<<1)
+	s.build(a, m+1, r, n<<1|1)
+	s.seg[n] = s.seg[n<<1] + s.seg[n<<1|1]
+}
+
+// Update value at index x
+func (s segmentTree) update(x, v, l, r, n int) {
+	if l == r {
+		s.seg[n] = v
+		return
+	}
+	m := (l + r) >> 1
+	if x <= m {
+		s.update(x, v, l, m, n<<1)
+	} else {
+		s.update(x, v, m+1, r, n<<1|1)
+	}
+	s.seg[n] = s.seg[n<<1] + s.seg[n<<1|1]
+}
+
+// Find element with prefix sum v
+func (s segmentTree) query(v, l, r, n int) (int, int) {
+	if l == r {
+		return s.seg[n], l
+	}
+	m := (l + r) >> 1
+	if s.seg[n<<1] >= v {
+		return s.query(v, l, m, n<<1)
+	} else {
+		return s.query(v-s.seg[n<<1], m+1, r, n<<1|1)
+	}
+}