USACO 1.4 Arithmetic Progressions
按步长递增的顺序搜索，先预处理出所有可能的步长，及位置值（用flg[]标记），再深搜
预处理稍微有点麻烦
code
1 /*
2 ID: superbi1
3 LANG: C++
4 TASK: ariprog
5 */
6 #include <iostream>
7 #include <fstream>
8 #include <string>
9 #include <stdlib.h>
10 #define NL 250
11 using namespace std;
12
13 int f0[NL*NL], nf; //起始点
14 int dis[NL*NL], nd; //步长
15 bool flg[NL*NL*2+1], flg1[NL*NL*2+1];
16 bool yes;
17
18 ifstream fin("ariprog.in");
19 ofstream fout("ariprog.out");
20
21 int cmp(const void *a, const void *b)
22 {
23 return *(int *)a - *(int *)b;
24 }
25
26 void init(int m)
27 {
28 int i, j, t;
29 nf = 0;
30 fill_n(flg, m*m*2, 0);
31 for (i=0; i<=m; i++) {
32 for (j=0; j<=m; j++) {
33 t = i*i+j*j;
34 if (!flg[t]) {
35 flg[t] = 1;
36 f0[nf++] = t;
37 }
38 }
39 }
40 qsort(f0, nf, sizeof(f0[0]), cmp);
41 fill_n(flg1, m*m*2, 0);
42 nd = 0;
43 for (i=0; i<nf; i++) {
44 for (j=i+1; j<nf; j++) {
45 t = f0[j]-f0[i];
46 if (t > 4167) break;
47 if (!flg1[t]) {
48 flg1[t] = 1;
49 dis[nd++] = t;
50 }
51 }
52 }
53 qsort(dis, nd, sizeof(dis[0]), cmp);
54 }
55
56 void DFS(int a, int b, int lev, int n)
57 {
58 if (lev == n) {
59 yes = true;
60 fout << a << ' ' << b << endl;
61 return;
62 }
63 if (flg[a+b*lev]) DFS(a, b, lev+1, n);
64 }
65
66 void search(int n, int m)
67 {
68 int i, j;
69 int a, b;
70 for (i=0; i<nd; i++) {
71 if (dis[i]*2 > m*m*2) break;
72 for (j=0; j<nf; j++) {
73 a = f0[j];
74 b = dis[i];
75 if (a+2*b > m*m*2) break;
76 DFS(a, b, 0, n);
77 }
78 }
79 }
80
81 int main()
82 {
83 int N, M;
84 yes = false;
85 fin >> N >> M;
86 init(M);
87 search(N, M);
88 if (!yes) fout << "NONE" << endl;
89 return 0;
90 }
