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diff --git a/307/main.py b/307/main.py
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+++ b/307/main.py
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+#!/usr/bin/env python3
+"""307"""
+
+from typing import List
+
+
+class NumArray_Naive:
+    """Naive implementation. Useless."""
+
+    def __init__(self, nums: List[int]):
+        self.nums = nums
+
+    def update(self, index: int, val: int) -> None:
+        """Update."""
+        self.nums[index] = val
+
+    def sumRange(self, left: int, right: int) -> int:
+        """sumRange."""
+        return sum(self.nums[left : right + 1])
+
+
+class NumArray:
+    """Segment Tree implementation."""
+
+    def __init__(self, nums: List[int]):
+        self.nums = nums
+        self.n = len(nums)
+        self.segment_tree = [0] * (len(nums) * 2)
+        self.build_segment_tree()
+
+    def build_segment_tree(self):
+        """build segment tree."""
+        for i, j in zip(range(self.n, 2 * self.n), range(0, len(self.nums))):
+            self.segment_tree[i] = self.nums[j]
+        for i in reversed(range(1, self.n)):
+            self.segment_tree[i] = (
+                self.segment_tree[i * 2] + self.segment_tree[i * 2 + 1]
+            )
+        # print(self.segment_tree)
+
+    def update_segment_tree(self, index: int, val: int):
+        """update segment tree."""
+        pos: int = index + self.n
+        self.segment_tree[pos] = val
+        while pos > 0:
+            left = pos
+            right = pos
+            if pos % 2 == 0:
+                right = pos + 1
+            else:
+                left = pos - 1
+            self.segment_tree[int(pos / 2)] = (
+                self.segment_tree[left] + self.segment_tree[right]
+            )
+            pos = int(pos / 2)
+
+    def update(self, index: int, val: int) -> None:
+        """Update."""
+        self.update_segment_tree(index, val)
+
+    def sumRange(self, left: int, right: int) -> int:
+        """sumRange."""
+        l = left + self.n
+        r = right + self.n
+        s: int = 0
+
+        while l <= r:
+            if (l % 2) == 1:
+                s += self.segment_tree[l]
+                l += 1
+            if (r % 2) == 0:
+                s += self.segment_tree[r]
+                r -= 1
+            l = int(l / 2)
+            r = int(r / 2)
+
+        return s
+
+
+# Your NumArray object will be instantiated and called as such:
+# obj = NumArray(nums)
+# obj.update(index,val)
+# param_2 = obj.sumRange(left,right)
+
+
+def main():
+    """Main."""
+    num_array = NumArray([1, 3, 5])
+    print(num_array.sumRange(0, 2))
+    num_array.update(1, 2)
+    print(num_array.sumRange(0, 2))
+
+
+if __name__ == "__main__":
+    main()