Improved task 307

src/main/java/g0301_0400/s0307_range_sum_query_mutable/NumArray.java

@@ -1,43 +1,52 @@
 package g0301_0400.s0307_range_sum_query_mutable;
 
 // #Medium #Array #Design #Segment_Tree #Binary_Indexed_Tree
-// #2022_07_07_Time_99_ms_(88.96%)_Space_138.9_MB_(5.17%)
+// #2023_03_31_Time_92_ms_(90.27%)_Space_75.3_MB_(16.68%)
 
 public class NumArray {
+    private int[] tree;
     private int[] nums;
-    private int sum;
 
     public NumArray(int[] nums) {
+        tree = new int[nums.length + 1];
         this.nums = nums;
-        sum = 0;
-        for (int num : nums) {
-            sum += num;
+        // copy the array into the tree
+        System.arraycopy(nums, 0, tree, 1, nums.length);
+        for (int i = 1; i < tree.length; i++) {
+            int parent = i + (i & -i);
+            if (parent < tree.length) {
+                tree[parent] += tree[i];
+            }
         }
     }
 
     public void update(int index, int val) {
-        sum -= nums[index] - val;
+        int currValue = nums[index];
         nums[index] = val;
+        index++;
+        while (index < tree.length) {
+            tree[index] = tree[index] - currValue + val;
+            index = index + (index & -index);
+        }
     }
 
-    public int sumRange(int left, int right) {
-        int sumRange = 0;
-        if ((right - left) < nums.length / 2) {
-            // Array to sum is less than half
-            for (int i = left; i <= right; i++) {
-                sumRange += nums[i];
-            }
-        } else {
-            // Array to sum is more than half
-            // Better to take total sum and substract the numbers not in range
-            sumRange = sum;
-            for (int i = 0; i < left; i++) {
-                sumRange -= nums[i];
-            }
-            for (int i = right + 1; i < nums.length; i++) {
-                sumRange -= nums[i];
-            }
+    private int sum(int i) {
+        int sum = 0;
+        while (i > 0) {
+            sum += tree[i];
+            i -= (i & -i);
         }
-        return sumRange;
+        return sum;
+    }
+
+    public int sumRange(int left, int right) {
+        return sum(right + 1) - sum(left);
     }
 }
+
+/*
+ * Your NumArray object will be instantiated and called as such:
+ * NumArray obj = new NumArray(nums);
+ * obj.update(index,val);
+ * int param_2 = obj.sumRange(left,right);
+ */