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[25][25], ans = 1;
string x, y;

void init(int m)
{
    c[0][0] = 1;
    for (int i = 1; i <= m; i++)
    {
        c[i][0] = 1;
        for (int j = 1; j <= i; j++)
            c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
    }
}

void dfs(int lx, int rx, int ly, int ry)
{
    if (lx > rx) return;
    int cnt = 0; 
    while (lx <= rx)
    {
        int pos = ly;
        while (pos < ry && y[pos] != x[lx]) pos++;
        dfs(lx + 1, lx + pos - ly, ly, pos - 1); 
        lx += pos - ly + 1;
        ly = pos + 1;
        cnt++;
    }
    ans *= c[m][cnt];
}

int main()
{
    init(21);
    while (cin >> m, m)
    {
        ans = 1;
        cin >> x >> y;
        int len = x.length();
        dfs(1, len - 1, 0, len - 2);
        cout << ans << endl;
    }
    return 0;
}