Leetcode 1074 Number of Submatrices That Sum to Target

Author: jiajionline

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;
- }
+ }
 }