feat: Implement segment tree

README.md

@@ -38,6 +38,7 @@ Gradle is used for development.
   - [Monotonic Queue](src/main/java/dataStructures/queue/monotonicQueue)
 - Segment Tree
 - [Stack](src/main/java/dataStructures/stack)
+- [Segment Tree](src/main/java/dataStructures/segmentTree)
 - [Trie](src/main/java/dataStructures/trie)
 
 ## Algorithms
@@ -86,6 +87,7 @@ Gradle is used for development.
     * [AVL-tree](src/main/java/dataStructures/avlTree)
     * [Trie](src/main/java/dataStructures/trie)
     * [B-Tree](src/main/java/dataStructures/bTree)
+    * [Segment Tree](src/main/java/dataStructures/segmentTree) (Not covered in CS2040s but useful!)
     * Red-Black Tree (Not covered in CS2040s but useful!)
     * [Orthogonal Range Searching](src/main/java/algorithms/orthogonalRangeSearching)
     * Interval Trees (**WIP**)

src/main/java/dataStructures/heap/README.md

@@ -27,6 +27,20 @@ That said, in practice, the array-based implementation of a heap often provides
 former, in cache efficiency and memory locality. This is due to its contiguous memory layout. As such,
 the implementation shown here is a 0-indexed array-based heap.
 
+#### Obtain index representing child nodes
+Suppose the parent node is captured at index *i* of the array (1-indexed).
+**1-indexed**: <br>
+Left Child: *i* x 2 <br>
+Right Child:  *i* x 2 + 1 <br>
+
+The 1-indexed calculation is intuitive. So, when dealing with 0-indexed representation (as in our implementation), 
+one option is to convert 0-indexed to 1-indexed representation, do the above calculations, and revert. <br>
+(Note: Now, we assume parent node is captured at index *i* (0-indexed))
+
+**0-indexed**: <br>
+Left Child: (*i* + 1) x 2 - 1  = *i* x 2 + 1 <br>
+Right Child: (*i* + 1) x 2 + 1 - 1 = *i* x 2 + 2 <br>
+
 ### Relevance of increaseKey and decreaseKey operations
 
 The decision not to include explicit "decrease key" and "increase key" operations in the standard implementations of

src/main/java/dataStructures/segmentTree/README.md

@@ -0,0 +1,89 @@
+# Segment Tree
+
+## Background
+Segment Trees are primarily used to solve problems that require answers to queries on intervals of an array 
+with the possibility of modifying the array elements. 
+These queries could be finding the sum, minimum, or maximum in a subarray, or similar aggregated results.
+
+![Segment Tree](../../../../../docs/assets/images/SegmentTree.png)
+
+### Structure 
+(Note: See below for a brief description of the array-based implementation of a segment tree)
+
+A Segment Tree for an array of size *n* is a binary tree that stores information about segments of the array.
+Each node in the tree represents an interval of the array, with the root representing the entire array. 
+The structure satisfies the following properties:
+1. Leaf Nodes: Each leaf node represents a single element of the array.
+2. Internal Nodes: Each internal node represents the sum of the values of its children 
+(which captures the segment of the array). Summing up, this node captures the whole segment.
+3. Height: The height of the Segment Tree is O(log *n*), making queries and updates efficient.
+
+## Complexity Analysis
+**Time**: O(log(n)) in general for query and update operations,
+except construction which takes O(nlogn)
+
+**Space**: O(n), note for an array-based implementation, the array created should have size 4n (explained later)
+
+where n is the number of elements in the array.
+
+## Operations
+### Construction
+The construction of a Segment Tree starts with the root node representing the entire array and 
+recursively dividing the array into two halves until each segment is reduced to a single element. 
+This process is a divide-and-conquer strategy:
+1. Base Case: If the current segment of the array is reduced to a single element, create a leaf node.
+2. Recursive Case: Otherwise, split the array segment into two halves, construct the left and right children, 
+and then merge their results to build the parent node.
+
+This takes O(nlogn). logn in depth, and will visit each leaf node (number of leaf nodes could be roughly 2n) once.
+
+### Querying
+To query an interval, say to find the sum of elements in the interval (L, R), 
+the tree is traversed starting from the root:
+1. If the current node's segment is completely within (L, R), its value is part of the answer.
+2. If the current node's segment is completely outside (L, R), it is ignored.
+3. If the current node's segment partially overlaps with (L, R), the query is recursively applied to its children.
+
+This approach ensures that each level of the tree is visited only once, time complexity of O(logn).
+
+### Updating
+Updating an element involves changing the value of a leaf node and then propagating this change up to the root 
+to ensure the tree reflects the updated array. 
+This is done by traversing the path from the leaf node to the root 
+and updating each node along this path (update parent to the sum of its children). 
+
+This can be done in O(logn).
+
+## Array-based Segment Tree
+The array-based implementation of a Segment Tree is an efficient way to represent the tree in memory, especially 
+since a Segment Tree is a complete binary tree. 
+This method utilizes a simple array where each element of the array corresponds to a node in the tree, 
+including both leaves and internal nodes.
+
+### Why 4n space
+The size of the array needed to represent a Segment Tree for an array of size *n* is 2*2^ceil(log2(*n*)) - 1.
+We do 2^(ceil(log2(*n*))) because *n* might not be a perfect power of 2, 
+**so we expand the array size to the next power of 2**.
+This adjustment ensures that each level of the tree is fully filled except possibly for the last level, 
+which is filled from left to right.
+
+**BUT**, 2^(ceil(log2(*n*))) seems overly-complex. To ensure we have sufficient space, we can just consider 2*n
+because 2*n >= 2^(ceil(log2(*n*))).
+Now, these 2n nodes can be thought of as the 'leaf' nodes (or more precisely, an upper-bound). To account for the 
+intermediate nodes, we use the property that for a complete binary that is fully filled, the number of leaf nodes 
+= number of intermediate nodes (recall: sum i -> 0 to n-1 of 2^i = 2^n). So we create an array of size 2n * 2 = 4n to 
+guarantee we can house the entire segment tree.
+
+### Obtain index representing child nodes
+Suppose the parent node is captured at index *i* of the array (1-indexed).
+**1-indexed**: <br>
+Left Child: *i* x 2 <br>
+Right Child:  *i* x 2 + 1 <br>
+
+The 1-indexed calculation is intuitive. So, when dealing with 0-indexed representation (as in our implementation),
+one option is to convert 0-indexed to 1-indexed representation, do the above calculations, and revert. <br>
+(Note: Now, we assume parent node is captured at index *i* (0-indexed))
+
+**0-indexed**: <br>
+Left Child: (*i* + 1) x 2 - 1  = *i* x 2 + 1 <br>
+Right Child: (*i* + 1) x 2 + 1 - 1 = *i* x 2 + 2 <br>

src/main/java/dataStructures/segmentTree/SegmentTree.java

@@ -0,0 +1,115 @@
+package dataStructures.segmentTree;
+
+/**
+ * Implementation of a Segment Tree. Uses SegmentTreeNode as a helper node class.
+ */
+public class SegmentTree {
+    private SegmentTreeNode root;
+    private int[] array;
+
+    /**
+     * Helper node class. Used internally.
+     */
+    private class SegmentTreeNode {
+        private SegmentTreeNode leftChild;   // left child
+        private SegmentTreeNode rightChild;  // right child
+        private int start; // start idx of range captured
+        private int end; // end idx of range captured
+        private int sum; // sum of all elements between start and end index inclusive
+
+        /**
+         * Constructor
+         * @param leftChild
+         * @param rightChild
+         * @param start
+         * @param end
+         * @param sum
+         */
+        public SegmentTreeNode(SegmentTreeNode leftChild, SegmentTreeNode rightChild, int start, int end, int sum) {
+            this.leftChild = leftChild;
+            this.rightChild = rightChild;
+            this.start = start;
+            this.end = end;
+            this.sum = sum;
+        }
+    }
+
+    /**
+     * Constructor.
+     * @param nums
+     */
+    public SegmentTree(int[] nums) {
+        root = buildTree(nums, 0, nums.length - 1);
+        array = nums;
+    }
+
+    private SegmentTreeNode buildTree(int[] nums, int start, int end) {
+        if (start == end) {
+            return new SegmentTreeNode(null, null, start, end, nums[start]);
+        }
+        int mid = start + (end - start) / 2;
+        SegmentTreeNode left = buildTree(nums, start, mid);
+        SegmentTreeNode right = buildTree(nums, mid + 1, end);
+        return new SegmentTreeNode(left, right, start, end, left.sum + right.sum);
+    }
+
+    /**
+     * Queries the sum of all values in the specified range.
+     * @param leftEnd
+     * @param rightEnd
+     * @return the sum.
+     */
+    public int query(int leftEnd, int rightEnd) {
+        return query(root, leftEnd, rightEnd);
+    }
+
+    private int query(SegmentTreeNode node, int leftEnd, int rightEnd) {
+        // this is the case when:
+        //                start     end
+        // range query:  ^             ^  --> so simply capture the sum at this node!
+        if (leftEnd <= node.start && node.end <= rightEnd) {
+            return node.sum;
+        }
+        int rangeSum = 0;
+        int mid = node.start + (node.end - node.start) / 2;
+        // Consider the 3 possible kinds of range queries
+        //           start          mid          end
+        // poss 1:         ^      ^
+        // poss 2:             ^            ^
+        // poss 3:                       ^      ^
+        if (leftEnd <= mid) {
+            rangeSum += query(node.leftChild, leftEnd, Math.min(rightEnd, mid)); // poss1 or poss2
+        }
+        if (mid + 1 <= rightEnd) {
+            rangeSum += query(node.rightChild, Math.max(leftEnd, mid + 1), rightEnd); // poss2 or poss3
+        }
+        return rangeSum;
+    }
+
+    /**
+     * Updates the segment tree based on updates to the array at the specified index with the specified value.
+     * @param idx
+     * @param val
+     */
+    public void update(int idx, int val) {
+        if (idx > array.length) {
+            return;
+        }
+        array[idx] = val;
+        update(root, idx, val);
+    }
+
+    private void update(SegmentTreeNode node, int idx, int val) {
+        if (node.start == node.end && node.start == idx) {
+            node.sum = val; // node is holding a single value; now updated
+            return;
+        }
+        int mid = node.start + (node.end - node.start) / 2;
+        if (idx <= mid) {
+            update(node.leftChild, idx, val);
+        } else {
+            update(node.rightChild, idx, val);
+        }
+        node.sum = node.leftChild.sum + node.rightChild.sum; // propagate updates up
+    }
+}

src/main/java/dataStructures/segmentTree/arrayRepresentation/SegmentTree.java

@@ -0,0 +1,105 @@
+package dataStructures.segmentTree.arrayRepresentation;
+
+/**
+ * Array-based implementation of a Segment Tree.
+ */
+public class SegmentTree {
+    private int[] tree;
+    private int[] array;
+
+    /**
+     * Constructor.
+     * @param nums
+     */
+    public SegmentTree(int[] nums) {
+        tree = new int[4 * nums.length];  // Need to account for up to 4n nodes.
+        array = nums;
+        buildTree(nums, 0, nums.length - 1, 0);
+    }
+
+    /**
+     * Builds the tree from the given array of numbers.
+     * Unlikely before where we capture child nodes in the helper node class, here we capture position of child nodes
+     * in the array-representation of the tree with an additional variable.
+     * @param nums
+     * @param start
+     * @param end
+     * @param idx tells us which index of the tree array we are at.
+     */
+    private void buildTree(int[] nums, int start, int end, int idx) {
+        // recall, each node is a position in the array
+        // explicitly track which position in the array to fill with idx variable
+        if (start == end) {
+            tree[idx] = nums[start];
+            return;
+        }
+        int mid = start + (end - start) / 2;
+        int idxLeftChild = (idx + 1) * 2 - 1; // convert from 0-based to 1-based, do computation, then revert
+        buildTree(nums, start, mid, idxLeftChild);
+        int idxRightChild = (idx + 1) * 2 + 1 - 1; // convert from 0-based to 1-based, do computation, then revert
+        buildTree(nums, mid + 1, end, idxRightChild);
+        tree[idx] = tree[idxLeftChild] + tree[idxRightChild];
+    }
+
+    /**
+     * Queries the sum of all values in the specified range.
+     * @param leftEnd
+     * @param rightEnd
+     * @return the sum.
+     */
+    public int query(int leftEnd, int rightEnd) {
+        return query(0, 0, array.length - 1, leftEnd, rightEnd);
+    }
+
+    private int query(int nodeIdx, int startRange, int endRange, int leftEnd, int rightEnd) {
+        // this is the case when:
+        //                start     end
+        // range query:  ^             ^  --> so simply capture the sum at this node!
+        if (leftEnd <= startRange && endRange <= rightEnd) {
+            return tree[nodeIdx];
+        }
+        int rangeSum = 0;
+        int mid = startRange + (endRange - startRange) / 2;
+        // Consider the 3 possible kinds of range queries
+        //           start          mid          end
+        // poss 1:         ^      ^
+        // poss 2:             ^            ^
+        // poss 3:                       ^      ^
+        if (leftEnd <= mid) {
+            int idxLeftChild = (nodeIdx + 1) * 2 - 1;
+            rangeSum += query(idxLeftChild, startRange, mid, leftEnd, Math.min(rightEnd, mid));
+        }
+        if (mid + 1 <= rightEnd) {
+            int idxRightChild = (nodeIdx + 1) * 2 + 1 - 1;
+            rangeSum += query(idxRightChild, mid + 1, endRange, Math.max(leftEnd, mid + 1), rightEnd);
+        }
+        return rangeSum;
+    }
+
+    /**
+     * Updates the segment tree based on updates to the array at the specified index with the specified value.
+     * @param idx
+     * @param val
+     */
+    public void update(int idx, int val) {
+        if (idx > array.length) {
+            return;
+        }
+        array[idx] = val;
+        update(0, 0, array.length - 1, idx, val);
+    }
+
+    private void update(int nodeIdx, int startRange, int endRange, int idx, int val) {
+        if (startRange == endRange) {
+            tree[nodeIdx] = val;
+            return;
+        }
+        int mid = startRange + (endRange - startRange) / 2;
+        if (idx <= mid) {
+            update(nodeIdx * 2 + 1, startRange, mid, idx, val);
+        } else {
+            update(nodeIdx * 2 + 2, mid + 1, endRange, idx, val);
+        }
+        tree[nodeIdx] = tree[nodeIdx * 2 + 1] + tree[nodeIdx * 2 + 2];
+    }
+}

src/test/java/dataStructures/segmentTree/SegmentTreeTest.java

@@ -0,0 +1,39 @@
+package dataStructures.segmentTree;
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class SegmentTreeTest {
+    @Test
+    public void construct_shouldConstructSegmentTree() {
+        int[] arr1 = new int[] {7, 77, 37, 67, 33, 73, 13, 2, 7, 17, 87, 53};
+        SegmentTree tree1 = new SegmentTree(arr1);
+        assertEquals(arr1[1] + arr1[2] + arr1[3], tree1.query(1, 3));
+        assertEquals(arr1[4] + arr1[5] + arr1[6] + arr1[7], tree1.query(4, 7));
+        int sum1 = 0;
+        for (int i = 0; i < arr1.length; i++) {
+            sum1 += arr1[i];
+        }
+        assertEquals(sum1, tree1.query(0, arr1.length - 1));
+
+
+        int[] arr2 = new int[] {7, -77, 37, 67, -33, 0, 73, -13, 2, -7, 17, 0, -87, 53, 0}; // some negatives and 0s
+        SegmentTree tree2 = new SegmentTree(arr1);
+        assertEquals(arr1[1] + arr1[2] + arr1[3], tree2.query(1, 3));
+        assertEquals(arr1[4] + arr1[5] + arr1[6] + arr1[7], tree2.query(4, 7));
+        int sum2 = 0;
+        for (int i = 0; i < arr1.length; i++) {
+            sum2 += arr1[i];
+        }
+        assertEquals(sum2, tree2.query(0, arr1.length - 1));
+    }
+
+    @Test
+    public void update_shouldUpdateSegmentTree() {
+        int[] arr = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        SegmentTree tree = new SegmentTree(arr);
+        assertEquals(55, tree.query(0, 10));
+        tree.update(5, 55);
+        assertEquals(105, tree.query(0, 10));
+    }
+}