Add solution to day 20

06- June/20- K Radius Subarray Averages/20- K Radius Subarray Averages (Ahmed Gamal).cpp

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+// Author: Ahmed Gamal
+
+// for this problem, we can use prefix sum to calculate the sum of each subarray in O(1) time
+// then we can just iterate over the array and calculate the average of each subarray
+
+class Solution {
+public:
+    vector<int> getAverages(vector<int>& nums, int k) {
+        // ans[i] = average of subarray [i - k, i + k], if i - k < 0 or i + k >= nums.size() then ans[i] = -1
+        // prefix[i] = sum of subarray [0, i - 1]
+        vector<int> ans(nums.size(), -1);
+        vector<long long> prefix(nums.size() + 1);
+
+        // calculate prefix sum
+        for(int i = 1; i < prefix.size(); i++) {
+            prefix[i] += nums[i - 1] + prefix[i - 1];
+        }
+
+        // calculate the average of each subarray (from k + 1 to nums.size() - k)
+        for(int i = k + 1; i + k < prefix.size(); i++) {
+            ans[i - 1] = (prefix[i + k] - prefix[i - k - 1]) / (k << 1 | 1);
+        }
+
+        return ans;
+    }
+};