refactory the code

Author: haoel

algorithms/cpp/rangeSumQuery-Immutable/rangeSumQuery-Immutable.cpp

@@ -20,32 +20,29 @@
  ***************************************************************************************/
 
 class NumArray {
-public:
 /* 
  * Solution
  * =========
  *
  * The sum of all the elements starting from position 0 to position i is stored in
  * sums[i]. This way we can reconstruct the sum from position i to position j by
  * subtracting sums[i - 1] from sums[j], leaving us with the sum of the desired elements.
+ *
+ * Note: we can add a dummy sum at then beginning to simplify the code
  * 
  */
+private:
+    int size;
     vector <long long> sums;
-    NumArray(vector<int> &nums) {
-        if(nums.size())
-        {
-            sums.push_back(nums[0]);
-            for(int i = 1; i < nums.size(); i++)
-                sums.push_back(sums[i-1] + nums[i]);
-        }
+public:
+    NumArray(vector<int> &nums): size(nums.size()), sums(size+1, 0) {
+        for(int i=0; i<size; i++) {
+            sums[i+1] = sums[i] + nums[i];
+        } 
     }
     int sumRange(int i, int j) {
-        if(i > 0)
-            return sums[j] - sums[i - 1];
-        else
-            return sums[j];
+        return sums[j+1] - sums[i];
     }
-
 };