Link: https://leetcode.cn/problems/sparse-matrix-multiplication/

Question

difficulty: mid
adj diff: 3

Given two sparse matrices mat1 of size m x k and mat2 of size k x n, return the result of mat1 x mat2. You may assume that multiplication is always possible.

Example 1:

Input: mat1 = [[1,0,0],[-1,0,3]], mat2 = [[7,0,0],[0,0,0],[0,0,1]]
Output: [[7,0,0],[-7,0,3]]
Example 2:

Input: mat1 = [[0]], mat2 = [[0]]
Output: [[0]]

Constraints:

m == mat1.length
k == mat1[i].length == mat2.length
n == mat2[i].length
1 <= m, n, k <= 100
-100 <= mat1[i][j], mat2[i][j] <= 100

Code

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public int[][] multiply(int[][] mat1, int[][] mat2) {
    int m = mat1.length;
    int k = mat2.length;
    int n = mat2[0].length;
    int[][] ans = new int[m][n];

    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            // ans[i][j] = multiply mat1[i] with mat2 column j
            ans[i][j] = 0;
            for (int h = 0; h < k; h++) {
                ans[i][j] += mat1[i][h] * mat2[h][j];
            }
        }
    }
    return ans;
}