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Author: knockcat

1074-number-of-submatrices-that-sum-to-target/1074-number-of-submatrices-that-sum-to-target.cpp

@@ -0,0 +1,45 @@
+class Solution {
+public:
+    int numSubmatrixSumTarget(vector<vector<int>>& matrix, int target) {
+        
+        int m = matrix.size();
+        int n = matrix[0].size();
+        
+        // prefix sum
+        
+        for(int row = 0; row < m; ++row)
+        {
+            for(int col = 1; col < n; ++col)
+            {
+                matrix[row][col] += matrix[row][col-1];
+            }
+        }
+        
+        int count = 0;
+        
+        // fix 2 colums
+        
+        for(int c1 = 0; c1 < n; ++c1)
+        {
+            for(int c2 = c1; c2 < n; ++c2)
+            {
+                unordered_map<int,int> mp;
+                int sum = 0;
+                mp.insert({0,1});
+                
+                for(int row = 0; row < m; ++row)
+                {
+                    sum += matrix[row][c2] - (c1 > 0 ? matrix[row][c1-1] : 0);
+                    count += mp[sum-target];
+                    
+                    if(mp.find(sum) != mp.end())
+                        ++mp[sum];
+                    else
+                        mp[sum] = 1;
+                }
+            }
+        }
+        
+        return count;
+    }
+};