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'.

Input: matrix = [[0,1,0],[1,1,1],[0,1,0]], target = 0 Output: 4 Explanation: The four 1x1 submatrices that only contain 0. 

Input: matrix = [[1,-1],[-1,1]], target = 0 Output: 5 Explanation: The two 1x2 submatrices, plus the two 2x1 submatrices, plus the 2x2 submatrix. 

Input: matrix = [[904]], target = 0 Output: 0 

• 1 <= matrix.length <= 100
• 1 <= matrix[0].length <= 100
• -1000 <= matrix[i] <= 1000
• -10^8 <= target <= 10^8

use std::collections::HashMap;implSolution{pubfnnum_submatrix_sum_target(matrix:Vec<Vec<i32>>,target:i32) -> i32{let m = matrix.len();let n = matrix[0].len();letmut matrix = matrix;letmut count = HashMap::new();letmut ret = 0;for x1 in0..m {for x2 in x1..m { count.insert((x1, x2,0),1);}}for x2 in0..m {for y2 in0..n {if x2 > 0{ matrix[x2][y2] += matrix[x2 - 1][y2];}if y2 > 0{ matrix[x2][y2] += matrix[x2][y2 - 1];}if x2 > 0 && y2 > 0{ matrix[x2][y2] -= matrix[x2 - 1][y2 - 1];}for x1 in0..=x2 {letmut diff = matrix[x2][y2];if x1 > 0{ diff -= matrix[x1 - 1][y2];} ret += *count.get(&(x1, x2, diff - target)).unwrap_or(&0);*count.entry((x1, x2, diff)).or_insert(0) += 1;}}} ret }}