Problem
给你2*n的格子，每个格子有一个字母，从任意一点出发，不重复的经过上下左右，生成要求的字符串。问有几种不同的走法。
Solution
Notice
特殊情况下有重复。
Code
#include#include #include #include #include using namespace std;#define sqz main#define ll long long#define reg register int#define rep(i, a, b) for (reg i = a; i <= b; i++)#define per(i, a, b) for (reg i = a; i >= b; i--)#define travel(i, u) for (reg i = head[u]; i; i = edge[i].next)const int INF = 1e9, mod = 1e9 + 7, Ha = 826036489, N = 2000;const double eps = 1e-6, phi = acos(-1.0);ll modd(ll a, ll b) {if (a >= b || a < 0) a %= b; if (a < 0) a += b; return a;}ll read(){ ll x = 0; int zf = 1; char ch; while (ch != '-' && (ch < '0' || ch > '9')) ch = getchar();if (ch == '-') zf = -1, ch = getchar(); while (ch >= '0' && ch <= '9') x = x * 10 + ch - '0', ch = getchar(); return x * zf;}void write(ll y) { if (y < 0) putchar('-'), y = -y; if (y > 9) write(y / 10); putchar(y % 10 + '0');}char S[2][N + 5], st[N + 5];ll f[2][N + 5][N + 5], mi[N + 5];int n, m;void Calc(ll &X, ll Y){ X += Y; if (X >= mod) X -= mod;}struct Hash{ ll hash[N + 5]; void Make(int n, char *s) { hash[0] = 0; rep(i, 1, n) hash[i] = (hash[i - 1] * 31 + s[i] - 'a') % Ha; } ll Cut(int l, int r) { return (hash[r] - hash[l - 1] * mi[r - l + 1] % Ha + Ha) % Ha; }}pre[2], suf[2], Comp;ll Solve(int flag){ ll T = 0; memset(f, 0, sizeof(f)); rep(j, 1, n) { f[0][j][0] = f[1][j][0] = 1; rep(i, 0, 1) { rep(k, 2, min(n - j + 1, m / 2)) if (Comp.Cut(m - 2 * k + 1, m - k) == pre[i].Cut(j, j + k - 1) && Comp.Cut(m - k + 1, m) == suf[1 - i].Cut(n - (j + k - 1) + 1, n - j + 1)) if (2 * k != m || flag) Calc(T, f[i][j][m - 2 * k]); rep(k, 2, min(j, m / 2)) if (Comp.Cut(k + 1, 2 * k) == pre[i].Cut(j - k + 1, j) && Comp.Cut(1, k) == suf[1 - i].Cut(n - j + 1, n - (j - k + 1) + 1)) if (2 * k != m || flag) Calc(f[i][j + 1][2 * k], 1); } rep(i, 0, 1) rep(k, 0, m - 1) if (S[i][j] == st[k + 1]) { Calc(f[i][j + 1][k + 1], f[i][j][k]); if (k + 2 <= m && S[1 - i][j] == st[k + 2]) Calc(f[1 - i][j + 1][k + 2], f[i][j][k]); } rep(i, 0, 1) Calc(T, f[i][j + 1][m]); } return T;}int sqz(){ scanf("%s%s%s", S[0] + 1, S[1] + 1, st + 1); n = strlen(S[0] + 1), m = strlen(st + 1); mi[0] = 1; rep(i, 1, 2000) mi[i] = (mi[i - 1] * 31) % Ha; rep(i, 0, 1) { pre[i].Make(n, S[i]); reverse(S[i] + 1, S[i] + n + 1); suf[i].Make(n, S[i]); reverse(S[i] + 1, S[i] + n + 1); } Comp.Make(m, st); if (m == 1) { printf("%I64d\n", Solve(1) % mod); return 0; } ll ans = 0; Calc(ans, Solve(1)); reverse(st + 1, st + m + 1); Comp.Make(m, st); Calc(ans, Solve(0)); if (m == 2) rep(i, 1, n) { if (S[0][i] == st[1] && S[1][i] == st[2]) Calc(ans, mod - 1); if (S[1][i] == st[1] && S[0][i] == st[2]) Calc(ans, mod - 1); } printf("%I64d\n", ans); return 0;}