create 0304-range-sum-query-2d-immutable.cpp

Author: Maunish-dave

Diff for: cpp/0304-range-sum-query-2d-immutable.cpp

@@ -0,0 +1,37 @@
+class NumMatrix {
+public:
+
+    vector<vector<int>> dp;
+
+    NumMatrix(vector<vector<int>>& matrix) {
+        int r = matrix.size(),c = matrix[0].size();
+        dp = matrix;
+
+        for(int i =0;i<r;i++){
+            for(int j =0;j<c;j++){
+                if(i>0) dp[i][j] += dp[i-1][j]; // add prev row
+                if(j>0) dp[i][j] += dp[i][j-1]; // add prev col
+
+                // remove diagonal as it is added twice above
+                if(i>0&&j>0) dp[i][j] -= dp[i-1][j-1];
+            }
+        }
+    }
+    
+    int sumRegion(int row1, int col1, int row2, int col2) {
+        int answer = dp[row2][col2];
+
+        if(row1>0) answer -= dp[row1-1][col2];  // remove prev row on col1
+        if(col1>0) answer -= dp[row2][col1-1]; // remo prev col on row2
+
+        // add prev diagonal as pre row and prev col both contains that value
+        if(row1>0&&col1>0) answer += dp[row1-1][col1-1];
+        return answer;
+    }
+};
+
+/**
+ * Your NumMatrix object will be instantiated and called as such:
+ * NumMatrix* obj = new NumMatrix(matrix);
+ * int param_1 = obj->sumRegion(row1,col1,row2,col2);
+ */