LeetCode 311. Sparse Matrix Multiplication
原题链接在这里:https://leetcode.com/problems/sparse-matrix-multiplication/description/
题目:
Given two sparse matrices A and B, return the result of AB.
You may assume that A's column number is equal to B's row number.
Example:
A = [ [ 1, 0, 0], [-1, 0, 3] ] B = [ [ 7, 0, 0 ], [ 0, 0, 0 ], [ 0, 0, 1 ] ] | 1 0 0 | | 7 0 0 | | 7 0 0 | AB = | -1 0 3 | x | 0 0 0 | = | -7 0 3 | | 0 0 1 |
题解:
按照两个矩阵相乘的公式计算结果.
Since It is spase, skip k loop when A[i][j] == 0.
Time Complexity: O(m*n*o). m = A.length, n = A[0].length, o = B[0].length.
Space: O(1). regardless res.
AC Java:
1 class Solution { 2 public int[][] multiply(int[][] A, int[][] B) { 3 int m = A.length; 4 int n = A[0].length; 5 int o = B[0].length; 6 7 int [][] res = new int[m][o]; 8 for(int i = 0; i<m; i++){ 9 for(int j = 0; j<n; j++){ 10 if(A[i][j] != 0){ 11 for(int k = 0; k<o; k++){ 12 res[i][k] += A[i][j]*B[j][k]; 13 } 14 } 15 } 16 } 17 return res; 18 } 19 }
AC Ptyhon:
1 class Solution: 2 def multiply(self, mat1: List[List[int]], mat2: List[List[int]]) -> List[List[int]]: 3 m, n, o = len(mat1), len(mat1[0]), len(mat2[0]) 4 res = [[0 for _ in range(o)] for _ in range(m)] 5 6 for i in range(m): 7 for j in range(n): 8 if mat1[i][j] != 0: 9 for k in range(o): 10 res[i][k] += mat1[i][j] * mat2[j][k] 11 12 return res