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：

 #include <iostream>
 using namespace std;

 int m, c[][], ans = ;
 string x, y;

 void init(int m)
 {
     c[][] = ;
     for (int i = ; i <= m; i++)
     {
         c[i][] = ;
         for (int j = ; j <= i; j++)
             c[i][j] = c[i - ][j] + c[i - ][j - ];
     }
 }

 void dfs(int lx, int rx, int ly, int ry)
 {
     if (lx > rx) return;
     int cnt = ;
     while (lx <= rx)
     {
         int pos = ly;
         while (pos < ry && y[pos] != x[lx]) pos++;
         dfs(lx + , lx + pos - ly, ly, pos - );
         lx += pos - ly + ;
         ly = pos + ;
         cnt++;
     }
     ans *= c[m][cnt];
 }

 int main()
 {
     init();
     while (cin >> m, m)
     {
         ans = ;
         cin >> x >> y;
         int len = x.length();
         dfs(, len - , , len - );
         cout << ans << endl;
     }
     return ;
 }