Non-recursive Segment Tree implementation (#1543)

* Non-recursive Segment Tree implementation

* Added type hints and explanations links

data_structures/binary_tree/non_recursive_segment_tree.py

@@ -0,0 +1,153 @@
+"""
+A non-recursive Segment Tree implementation with range query and single element update,
+works virtually with any list of the same type of elements with a "commutative" combiner.
+
+Explanation:
+https://www.geeksforgeeks.org/iterative-segment-tree-range-minimum-query/
+https://www.geeksforgeeks.org/segment-tree-efficient-implementation/
+
+>>> SegmentTree([1, 2, 3], lambda a, b: a + b).query(0, 2)
+6
+>>> SegmentTree([3, 1, 2], min).query(0, 2)
+1
+>>> SegmentTree([2, 3, 1], max).query(0, 2)
+3
+>>> st = SegmentTree([1, 5, 7, -1, 6], lambda a, b: a + b)
+>>> st.update(1, -1)
+>>> st.update(2, 3)
+>>> st.query(1, 2)
+2
+>>> st.query(1, 1)
+-1
+>>> st.update(4, 1)
+>>> st.query(3, 4)
+0
+>>> st = SegmentTree([[1, 2, 3], [3, 2, 1], [1, 1, 1]], lambda a, b: [a[i] + b[i] for i in range(len(a))])
+>>> st.query(0, 1)
+[4, 4, 4]
+>>> st.query(1, 2)
+[4, 3, 2]
+>>> st.update(1, [-1, -1, -1])
+>>> st.query(1, 2)
+[0, 0, 0]
+>>> st.query(0, 2)
+[1, 2, 3]
+"""
+from typing import List, Callable, TypeVar
+
+T = TypeVar("T")
+
+
+class SegmentTree:
+    def __init__(self, arr: List[T], fnc: Callable[[T, T], T]) -> None:
+        """
+        Segment Tree constructor, it works just with commutative combiner.
+        :param arr: list of elements for the segment tree
+        :param fnc: commutative function for combine two elements
+
+        >>> SegmentTree(['a', 'b', 'c'], lambda a, b: '{}{}'.format(a, b)).query(0, 2)
+        'abc'
+        >>> SegmentTree([(1, 2), (2, 3), (3, 4)], lambda a, b: (a[0] + b[0], a[1] + b[1])).query(0, 2)
+        (6, 9)
+        """
+        self.N = len(arr)
+        self.st = [None for _ in range(len(arr))] + arr
+        self.fn = fnc
+        self.build()
+
+    def build(self) -> None:
+        for p in range(self.N - 1, 0, -1):
+            self.st[p] = self.fn(self.st[p * 2], self.st[p * 2 + 1])
+
+    def update(self, p: int, v: T) -> None:
+        """
+        Update an element in log(N) time
+        :param p: position to be update
+        :param v: new value
+
+        >>> st = SegmentTree([3, 1, 2, 4], min)
+        >>> st.query(0, 3)
+        1
+        >>> st.update(2, -1)
+        >>> st.query(0, 3)
+        -1
+        """
+        p += self.N
+        self.st[p] = v
+        while p > 1:
+            p = p // 2
+            self.st[p] = self.fn(self.st[p * 2], self.st[p * 2 + 1])
+
+    def query(self, l: int, r: int) -> T:
+        """
+        Get range query value in log(N) time
+        :param l: left element index
+        :param r: right element index
+        :return: element combined in the range [l, r]
+
+        >>> st = SegmentTree([1, 2, 3, 4], lambda a, b: a + b)
+        >>> st.query(0, 2)
+        6
+        >>> st.query(1, 2)
+        5
+        >>> st.query(0, 3)
+        10
+        >>> st.query(2, 3)
+        7
+        """
+        l, r = l + self.N, r + self.N
+        res = None
+        while l <= r:
+            if l % 2 == 1:
+                res = self.st[l] if res is None else self.fn(res, self.st[l])
+            if r % 2 == 0:
+                res = self.st[r] if res is None else self.fn(res, self.st[r])
+            l, r = (l + 1) // 2, (r - 1) // 2
+        return res
+
+
+if __name__ == "__main__":
+    from functools import reduce
+
+    test_array = [1, 10, -2, 9, -3, 8, 4, -7, 5, 6, 11, -12]
+
+    test_updates = {
+        0: 7,
+        1: 2,
+        2: 6,
+        3: -14,
+        4: 5,
+        5: 4,
+        6: 7,
+        7: -10,
+        8: 9,
+        9: 10,
+        10: 12,
+        11: 1,
+    }
+
+    min_segment_tree = SegmentTree(test_array, min)
+    max_segment_tree = SegmentTree(test_array, max)
+    sum_segment_tree = SegmentTree(test_array, lambda a, b: a + b)
+
+    def test_all_segments():
+        """
+        Test all possible segments
+        """
+        for i in range(len(test_array)):
+            for j in range(i, len(test_array)):
+                min_range = reduce(min, test_array[i : j + 1])
+                max_range = reduce(max, test_array[i : j + 1])
+                sum_range = reduce(lambda a, b: a + b, test_array[i : j + 1])
+                assert min_range == min_segment_tree.query(i, j)
+                assert max_range == max_segment_tree.query(i, j)
+                assert sum_range == sum_segment_tree.query(i, j)
+
+    test_all_segments()
+
+    for index, value in test_updates.items():
+        test_array[index] = value
+        min_segment_tree.update(index, value)
+        max_segment_tree.update(index, value)
+        sum_segment_tree.update(index, value)
+        test_all_segments()