Given a matrix, and a target, return the number of non-empty submatrices that sum to target.
A submatrix x1, y1, x2, y2 is the set of all cells matrix[x][y] with x1 <= x <= x2 and y1 <= y <= y2.
Two submatrices (x1, y1, x2, y2) and (x1', y1', x2', y2') are different if they have some coordinate that is different: for example, if x1 != x1'.
Example 1:
Input: matrix = [[0,1,0],[1,1,1],[0,1,0]], target = 0
Output: 4
Explanation: The four 1x1 submatrices that only contain 0.
Example 2:
Input: matrix = [[1,-1],[-1,1]], target = 0
Output: 5
Explanation: The two 1x2 submatrices, plus the two 2x1 submatrices, plus the 2x2 submatrix.
Note:
1 <= matrix.length <= 300
1 <= matrix[0].length <= 300
-1000 <= matrix[i] <= 1000
-10^8 <= target <= 10^8
public class Solution {
public int helper(int[] sums, int target) {
int sum = 0, ans = 0;
HashMap<Integer, Integer> map = new HashMap<>();
map.put(0, 1);
for (int x : sums) {
sum += x;
ans += map.getOrDefault(sum - target, 0);
map.put(sum, map.getOrDefault(sum, 0) + 1);
}
return ans;
}
public int numSubmatrixSumTarget(int[][] A, int target) {
if (A.length == 0)
return 0;
int ans = 0;
// Take 2 pointers
for (int i = 0; i < A[0].length; i++) {
int sum[] = new int[A.length];
for (int j = i; j < A[0].length; j++) {
// Find the prefix sum array of columns between these 2 pointers
for (int k = 0; k < A.length; k++)
sum[k] += A[k][j];
// Find subarays with sum == target
ans += helper(sum, target);
}
}
return ans;
}
}