Create matrix-block-sum.cpp

Author: kamyu104

C++/matrix-block-sum.cpp

@@ -0,0 +1,24 @@
+// Time:  O(m * n)
+// Space: O(m * n)
+
+class Solution {
+public:
+    vector<vector<int>> matrixBlockSum(vector<vector<int>>& mat, int K) {
+        int m = mat.size(), n = mat[0].size();
+        vector<vector<int>> accu(m + 1, vector<int>(n + 1));
+        for (int i = 0; i < m; ++i) {
+            for (int j = 0; j < n; ++j) {
+                accu[i + 1][j + 1] = accu[i + 1][j] + accu[i][j + 1] - accu[i][j] + mat[i][j];
+            }
+        }
+        vector<vector<int>> result(m, vector<int>(n));    
+        for (int i = 0; i < m; ++i) {
+            for (int j = 0; j < n; ++j) {
+                int r1 = max(i - K, 0), c1 = max(j - K, 0);
+                int r2 = min(i + K + 1, m), c2 = min(j + K + 1, n);
+                result[i][j] = accu[r2][c2] - accu[r1][c2] - accu[r2][c1] + accu[r1][c1];
+            }
+        }
+        return result;
+    }
+};