Leetcode 1074 Number of Submatrices That Sum to Target

1074_Number of Submatrices That Sum to Target/NumberofSubmatricesThatSumtoTarget2.java

@@ -1,30 +1,26 @@
 class Solution {
-  public int numSubmatrixSumTarget(int[][] matrix, int target) {
-    int m = matrix.length+1, n = matrix[0].length+1;
-
-    // compute 2D prefix sum with padding
-    int[][] ps = new int[m][n];
-    for (int i = 1; i < m; i++) {
-      for (int j = 1; j < n; j++) {
-        ps[i][j] = ps[i - 1][j] + ps[i][j - 1] - ps[i - 1][j - 1] + matrix[i - 1][j - 1];
-      }
-    }
+    public int numSubmatrixSumTarget(int[][] matrix, int target) {
+        int m = matrix.length+1, n = matrix[0].length+1;
+        int[][] presum = new int[m][n];
+        for(int i=1;i<m;i++) {
+            for(int j=1;j<n;j++) {
+                presum[i][j] = presum[i-1][j] + presum[i][j-1] - presum[i-1][j-1] + matrix[i-1][j-1];
+            }
+        }
 
-    int ans = 0, currSum;
-    Map<Integer, Integer> map = new HashMap();
-
-    for (int row = 1; row < m; row++) {
-      for (int row2 = row; row2 < m; row2++) {
-        map.clear();
-        map.put(0, 1);
-        for (int col = 1; col < n; col++) {
-          currSum = ps[row2][col] - ps[row - 1][col];
-          ans += map.getOrDefault(currSum - target, 0);
-          map.put(currSum, map.getOrDefault(currSum, 0) + 1);
+        int ans = 0;
+        for(int startRow=1;startRow<m;startRow++) {
+            for(int endRow=startRow;endRow<m;endRow++) {
+                Map<Integer,Integer> map = new HashMap<>();
+                map.put(0,1);
+                for(int col=1;col<n;col++) {
+                    int currSum = presum[endRow][col] - presum[startRow-1][col];
+                    ans += map.getOrDefault(currSum - target, 0);
+                    map.put(currSum, map.getOrDefault(currSum, 0)+1);
+                }
+            }
         }
-      }
-    }
+        return ans;
 
-    return ans;
-  }
+    }
 }