Create maximum-sum-of-3-non-overlapping-subarrays.cpp

Author: kamyu104

C++/maximum-sum-of-3-non-overlapping-subarrays.cpp

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+// Time:  O(n)
+// Space: O(n)
+
+class Solution {
+public:
+    vector<int> maxSumOfThreeSubarrays(vector<int>& nums, int k) {
+        const auto n = nums.size();
+        vector<int> accu = {0};
+        for (const auto& num : nums) {
+            accu.emplace_back(accu.back() + num);
+        }
+       
+        vector<int> left_pos(n);
+        for (int i = k, total = accu[k] - accu[0]; i < n; ++i) {
+            if (accu[i + 1] - accu[i + 1 - k] > total) {
+                left_pos[i] = i + 1 - k;
+                total = accu[i + 1] - accu[i + 1 - k];
+            } else { 
+                left_pos[i] = left_pos[i - 1];
+            }
+        }
+        
+        vector<int> right_pos(n, n - k);
+        for (int i = n - k - 1, total = accu[n] - accu[n - k]; i >= 0; --i) {
+            if (accu[i + k] - accu[i] > total) {
+                right_pos[i] = i;
+                total = accu[i + k] - accu[i];
+            } else {
+                right_pos[i] = right_pos[i + 1];
+            }
+        }
+        
+        vector<int> result(3);
+        for (int i = k, max_sum = 0; i <= n - 2 * k; ++i) {
+            auto left = left_pos[i - 1], right = right_pos[i + k];
+            auto total = (accu[i + k] - accu[i]) +
+                         (accu[left + k] - accu[left]) +
+                         (accu[right + k] - accu[right]);
+            if (total > max_sum) {
+                max_sum = total;
+                result = {left, i, right};
+            }
+        }
+        return result;
+    }
+};