Added segment tree in java

Author: Mad-Kat

Segment_trees/SegmentTree.java

@@ -0,0 +1,98 @@
+public class SegmentTree {
+
+    private long tree[];
+    private int length = 0;
+
+    //build the tree with new array size: 2*2^(log2(n))-1
+    public SegmentTree(long[] arr){
+        length = arr.length-1;
+        int x = (int) (Math.ceil(Math.log(arr.length) / Math.log(2)));
+        int max_size = 2 * (int) Math.pow(2, x) - 1;
+        tree = new long[max_size];
+        buildTree(arr, 0, length, 0);
+    }
+
+    public long getSum(int start, int end){
+        //input validation
+        if(start<0 || end > length || end<start){
+            System.out.println("Invalid Input");
+            return -1;
+        }
+        return getSum(0,length,start,end,0);
+    }
+
+    //recursive walking and summing in the tree
+    private long getSum(int segstart,int segend,int rangestart,int rangeend,int position){
+        //complete segment is in the range
+        if(rangestart<=segstart && rangeend>=segend){
+            return tree[position];
+        }
+
+        //segment is not in the range
+        if(rangestart>segend || rangeend<segstart){
+            return 0;
+        }
+
+        //segment is partially in the range
+        int mid = segstart + (segend-segstart)/2;
+
+        return getSum(segstart,mid,rangestart,rangeend,position*2+1) +
+                getSum(mid + 1,segend,rangestart,rangeend,position*2+2);
+    }
+
+    //recursive build of the tree in an array with the corresponding sum-values
+    private long buildTree(long[] arr, int segstart, int segend, int nodeposition){
+        //smallest segment (leaf)
+        if(segstart==segend){
+            tree[nodeposition] = arr[segstart];
+            return arr[segstart];
+        }
+
+        int mid = segstart + (segend-segstart)/2;
+        tree[nodeposition] = buildTree(arr,segstart,mid,nodeposition*2+1) +
+                buildTree(arr,mid+1,segend, nodeposition*2+2);
+        return tree[nodeposition];
+    }
+
+
+    public void updateValue(long[] arr, int position, long value){
+        //input validation
+        if (position < 0 || position > length - 1) {
+            System.out.println("Invalid Input");
+            return;
+        }
+        long diff = value - arr[position];
+        arr[position] = value;
+
+        updateValue(0,length,position,diff,0);
+    }
+
+    //recursive update all connected node to the value
+    private void updateValue(int segstart, int segend, int position, long diff, int node){
+        //actual position not in segment
+        if (position < segstart || position > segend)
+            return;
+
+        tree[node] = tree[node] + diff;
+        if(segend != segstart){
+            int mid = segstart + (segend-segstart) / 2;
+            updateValue(segstart,mid,position,diff,2*node+1);
+            updateValue(mid+1,segend,position,diff,2*node+2);
+        }
+    }
+
+    public static void main(String args[])
+    {
+        long arr[] = {1, 3, 4, 7, 11, 18};
+        SegmentTree  tree = new SegmentTree(arr);
+
+        System.out.println("Sum of values in given range = " +
+              tree.getSum(1, 3));
+
+        tree.updateValue(arr, 1, 10);
+
+        System.out.println("Updated sum of values in given range = " +
+            tree.getSum(1, 3));
+    }
+
+}