30.Range Sum Query 2D - Immutable

Author: Ibraam-Nashaat

30.Range Sum Query 2D - Immutable.cpp

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+
+// Problem Description: https://leetcode.com/problems/range-sum-query-2d-immutable/
+
+class NumMatrix {
+    vector<vector<int>> ans;
+public:
+    NumMatrix(vector<vector<int>>& matrix) {
+        int prefix = 0;
+        int above;
+        int n = matrix.size(), m = matrix[0].size();
+        ans.resize(n + 1, vector<int>(m + 1, 0));
+        for (int r = 0; r < n; r++)
+        {
+            prefix = 0;
+            for (int c = 0; c < m; c++)
+            {
+                prefix += matrix[r][c];
+                above = ans[r][c + 1];
+                ans[r + 1][c + 1] = prefix + above;
+            }
+        }
+    }
+
+    int sumRegion(int row1, int col1, int row2, int col2) {
+        row1++;
+        row2++;
+        col1++;
+        col2++;
+        int left, botright, above, leftabove;
+        botright = ans[row2][col2];
+        left = ans[row2][col1 - 1];
+        above = ans[row1 - 1][col2];
+        leftabove = ans[row1 - 1][col1 - 1];
+        return botright - above - left + leftabove;
+    }
+};
+
+/**
+ * Your NumMatrix object will be instantiated and called as such:
+ * NumMatrix* obj = new NumMatrix(matrix);
+ * int param_1 = obj->sumRegion(row1,col1,row2,col2);
+ */