java 311.稀疏矩阵乘法（方法＃1）.java

Posted 2021-05-08

public class Solution {
    public int[][] multiply(int[][] A, int[][] B) {
        if (A == null || A[0] == null || B == null || B[0] == null) return null;
        int m = A.length, n = A[0].length, l = B[0].length;
        int[][] C = new int[m][l];
        Map<Integer, HashMap<Integer, Integer>> tableB = new HashMap<>();
        
        for(int k = 0; k < n; k++) {
            tableB.put(k, new HashMap<Integer, Integer>());
            for(int j = 0; j < l; j++) {
                if (B[k][j] != 0){
                    tableB.get(k).put(j, B[k][j]);
                }
            }
        }

        for(int i = 0; i < m; i++) {
            for(int k = 0; k < n; k++) {
                if (A[i][k] != 0){
                    for (Integer j: tableB.get(k).keySet()) {
                        C[i][j] += A[i][k] * tableB.get(k).get(j);
                    }
                }
            }
        }
        return C;   
    }
}

public class Solution {
    public int[][] multiply(int[][] A, int[][] B) {
        if (A == null || A.length == 0 || B == null || B.length == 0) return new int[0][0];
        if (A[0].length != B.length) return new int[0][0];
        
        int m = A.length;
        int n = A[0].length;
        int nB = B[0].length;
        
        int[][] res = new int[m][nB];
        
        for (int i=0; i<m; i++) {
            for (int k=0; k<n; k++) {
                if (A[i][k] != 0) {
                    for (int j=0; j<nB; j++) {
                        if (B[k][j] != 0) res[i][j] += A[i][k]*B[k][j];
                    }
                }
            }
        }
        
        return res;
    }
}

public class Solution {

    public int[][] multiply(int[][] A, int[][] B) {
        int m = A.length, n = A[0].length, nB = B[0].length;
        int[][] result = new int[m][nB];

        List[] indexA = new List[m];
        for(int i = 0; i < m; i++) {
            List<Integer> numsA = new ArrayList<>();
            for(int j = 0; j < n; j++) {
                if(A[i][j] != 0){
                    numsA.add(j); 
                    numsA.add(A[i][j]);
                }
            }
            indexA[i] = numsA;
        }

        for(int i = 0; i < m; i++) {
            List<Integer> numsA = indexA[i];
            for(int p = 0; p < numsA.size() - 1; p += 2) {
                int colA = numsA.get(p);
                int valA = numsA.get(p + 1);
                for(int j = 0; j < nB; j ++) {
                    int valB = B[colA][j];
                    result[i][j] += valA * valB;
                }
            }
        }

        return result;   
    }
}