【HDOJ】2157 How many ways??

矩阵乘法，用DP做各种wa，后来发现原因了。
 1 #include <stdio.h>
 2 #include <string.h>
 3 
 4 typedef struct {
 5     int map[20][20];
 6 } matrix_st;
 7 
 8 matrix_st res, org;
 9 int n, m;
10 
11 matrix_st multiply(matrix_st a, matrix_st b) {
12     int i, j, k;
13     matrix_st c;
14 
15     for (i=0; i<n; ++i) {
16         for (j=0; j<n; ++j) {
17             c.map[i][j] = 0;
18             for (k=0; k<n; ++k)
19                 c.map[i][j] += a.map[i][k] * b.map[k][j];
20             c.map[i][j] %= 1000;
21         }
22     }
23 
24     return c;
25 }
26 
27 void calc(matrix_st org, int k) {
28     while (k) {
29         if (k & 1)
30             res = multiply(res, org);
31         k >>= 1;
32         org = multiply(org, org);
33     }
34 }
35 
36 int main() {
37     int t, i, j, k;
38 
39     while (scanf("%d %d", &n, &m)!=EOF && (n||m)) {
40         memset(org.map, 0, sizeof(org.map));
41         for (i=0; i<m; ++i) {
42             scanf("%d %d", &j, &k);
43             org.map[j][k] = 1;
44         }
45         scanf("%d", &t);
46         while (t--) {
47             memset(res.map, 0, sizeof(res.map));
48             for (i=0; i<n; ++i)
49                 res.map[i][i] = 1;
50             scanf("%d %d %d", &i, &j, &k);
51             calc(org, k);
52             printf("%d
", res.map[i][j]);
53         }
54 
55     }
56 
57     return 0;
58 }