poj1240 Pre-Post-erous!

思路：

根据前序序列和后序序列递归构造m叉树，确定每个节点的子节点数量。再用组合数公式累乘。

实现：

 1 #include <iostream>
 2 using namespace std;
 3
 4 int m, c[25][25], ans = 1;
 5 string x, y;
 6
 7 void init(int m)
 8 {
 9     c[0][0] = 1;
10     for (int i = 1; i <= m; i++)
11     {
12         c[i][0] = 1;
13         for (int j = 1; j <= i; j++)
14             c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
15     }
16 }
17
18 void dfs(int lx, int rx, int ly, int ry)
19 {
20     if (lx > rx) return;
21     int cnt = 0;
22     while (lx <= rx)
23     {
24         int pos = ly;
25         while (pos < ry && y[pos] != x[lx]) pos++;
26         dfs(lx + 1, lx + pos - ly, ly, pos - 1);
27         lx += pos - ly + 1;
28         ly = pos + 1;
29         cnt++;
30     }
31     ans *= c[m][cnt];
32 }
33
34 int main()
35 {
36     init(21);
37     while (cin >> m, m)
38     {
39         ans = 1;
40         cin >> x >> y;
41         int len = x.length();
42         dfs(1, len - 1, 0, len - 2);
43         cout << ans << endl;
44     }
45     return 0;
46 }