Segment Tree in Java

Contributors.md

@@ -38,3 +38,4 @@
 * ![Rashi Singh](https://github.com/RASHI3004)
 * ![Amrit Anwesh](https://github.com/shinozaki1595)
 * ![Vrutansh Patel](https://github.com/vrutansh)
+* ![Naresh Gupta](https://github.com/naresh1406)

segment_tree/SegmentTree.java

@@ -0,0 +1,92 @@
+class SegmentTree {
+    private int[] st;
+
+    public SegmentTree(int a[], int n) {
+        int x = (int) Math.ceil(Math.log(n) / Math.log(2));
+        int max_size = 2 * (int) Math.pow(2, x) - 1;
+        st = new int[max_size];
+        constructSTUtil(a, 0, n - 1, 0);
+    }
+
+    public int constructSTUtil(int a[], int s, int e, int index) {
+        if (s == e) {
+            st[index] = a[s];
+            return a[s];
+        }
+        int mid = s + (e - s) / 2;
+        st[index] = constructSTUtil(a, s, mid, 2 * index + 1) +
+                constructSTUtil(a, mid + 1, e, 2 * index + 2);
+        return st[index];
+    }
+
+    public int getSum(int n, int qs, int qe) {
+        // Check for erroneous input values
+        if (qs < 0 || qe > n - 1 || qs > qe) {
+            System.out.println("Invalid Input");
+            return -1;
+        }
+        return getSumUtil(0, n - 1, qs, qe, 0);
+    }
+
+    public int getSumUtil(int ss, int se, int qs, int qe, int si) {
+        // If segment of this node is a part of given range, then return the sum of the segment
+        if (qs <= ss && qe >= se)
+            return st[si];
+
+        // If segment of this node is outside the given range
+        if (se < qs || ss > qe)
+            return 0;
+
+        // If a part of this segment overlaps with the given range
+        int mid = ss + (se - ss) / 2;
+        return getSumUtil(ss, mid, qs, qe, 2 * si + 1) +
+                getSumUtil(mid + 1, se, qs, qe, 2 * si + 2);
+    }
+
+    public void updateValue(int arr[], int n, int i, int new_val) {
+        // Check for erroneous input index
+        if (i < 0 || i > n - 1) {
+            System.out.println("Invalid Input");
+            return;
+        }
+
+        // Get the difference between new value and old value
+        int diff = new_val - arr[i];
+
+        // Update the value in array
+        arr[i] = new_val;
+
+        // Update the values of nodes in segment tree
+        updateValueUtil(0, n - 1, i, diff, 0);
+    }
+
+    public void updateValueUtil(int ss, int se, int i, int diff, int si) {
+        // Base Case: If the input index lies outside the range of this segment
+        if (i < ss || i > se)
+            return;
+
+        // If the input index is in range of this node, then update the
+        // value of the node and its children
+        st[si] += diff;
+        if (se != ss) {
+            int mid = ss + (se - ss) / 2;
+            updateValueUtil(ss, mid, i, diff, 2 * si + 1);
+            updateValueUtil(mid + 1, se, i, diff, 2 * si + 2);
+        }
+    }
+
+    public static void main(String args[]) {
+        int arr[] = {1, 3, 5, 7, 9, 11};
+        int n = arr.length;
+        SegmentTree tree = new SegmentTree(arr, n);
+
+        // Print sum of values in array from index 1 to 3
+        System.out.println("Sum of values in given range[1,3] = " + tree.getSum(n, 1, 3));
+
+        // Update: set arr[1] = 10 and update corresponding segment tree nodes
+        tree.updateValue(arr, n, 1, 10);
+
+        // Find sum after the value is updated
+        System.out.println("Sum of values in given range[1,3] = " + tree.getSum(n, 1, 3));
+    }
+}