added new Java code (codedecks-in#21)

* added new Java code

* made changes to Range sum query

Author: Rudra407

Java/Range-Sum-Query-Mutable.java

@@ -0,0 +1,79 @@
+/**
+ * Your NumArray object will be instantiated and called as such:
+ * NumArray obj = new NumArray(nums);
+ * obj.update(i,val);
+ * int param_2 = obj.sumRange(i,j);
+ *
+ * Runtime : 11ms
+ * Memory : 45.4MB
+ * Logic : use segment tree for finding out the sum of the array at given range.
+ */
+
+
+class NumArray {
+
+    int[] tree;
+    int n;
+
+    public NumArray(int[] nums) {
+        if (nums.length > 0) {
+            n = nums.length;
+            tree = new int[n * 2];
+            buildTree(nums);
+        }
+    }
+    
+    // build the segment tree and store it in array of size 2 * nums[] length
+    // so the nums[] values are stores at leaves of the segment tree.
+    private void buildTree(int[] nums) {
+        for (int i = n, j = 0; i < 2 * n; i++, j++)
+            tree[i] = nums[j];
+        for (int i = n - 1; i > 0; --i)
+            tree[i] = tree[i * 2] + tree[i * 2 + 1];
+    }
+
+    // update the segment tree if new value to be inserted in tree, then update its parent at pos/2 and so on till root
+    void update(int pos, int val) {
+        pos += n;
+        tree[pos] = val;
+        while (pos > 0) {
+            int left = pos;
+            int right = pos;
+            if (pos % 2 == 0) {
+                right = pos + 1;
+            } else {
+                left = pos - 1;
+            }
+            // parent is updated after child is updated
+            tree[pos / 2] = tree[left] + tree[right];
+            pos /= 2;
+        }
+    }
+
+
+    // this is responsible for the sum betweeen given ranges
+    public int sumRange(int l, int r) {
+        // get leaf with value 'l'
+        l += n;
+        // get leaf with value 'r'
+        r += n;
+        int sum = 0;
+        while (l <= r) {
+            if ((l % 2) == 1) {
+                sum += tree[l];
+                l++;
+            }
+            if ((r % 2) == 0) {
+                sum += tree[r];
+                r--;
+            }
+            l /= 2;
+            r /= 2;
+        }
+        return sum;
+    }
+
+
+}
+
+

README.md

@@ -133,6 +133,8 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | #   | Title                                                                 | Solution                                  | Time   | Space  | Difficulty | Tag   | Note |
 | --- | --------------------------------------------------------------------- | ----------------------------------------- | ------ | ------ | ---------- | ----- | ---- |
 | 103 | [ZigZag Level Order](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/) | [JavaScript](./JavaScript/Binary-Tree-ZigZag-Traversal.js) | _O(n)_ | _O(n)_ | Medium       | Binary Tree |      |
+| 307 | [Range Sum Query - Mutable](https://leetcode.com/problems/range-sum-query-mutable/) | [Java](./Java/Range-Sum-Query-Mutable.java) | _O(logn)_ | _O(n)_ | Medium       | Segment Tree |      |
+
 
 <br/>
 <div align="right">