poj 1240 DFS

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不知道为什么分到DP分类里面去了，（pos1,pos2）表示一颗子数的前序遍历（pos3,pos4）表示该颗子数的后序遍历

然后可以将该子树分成更小的子树，直到pos1==pos2

每次的种类数就是子树的种类树相乘*C( M,子树个数)

AC代码如下：

#include <iostream>#include <cstring>#include <cstdio>#include <algorithm>using namespace std;char s1[100], s2[100];int M;int Cn( int n, int k ){    int ans = 1;    for( int i = 1; i <= k; i++ ){        ans *= n;        n--;    }    for( int i = 1; i <= k; i++ ){        ans /= i;    }    return ans;}int DFS( int pos1, int pos2, int pos3, int pos4 ){    if( pos1 == pos2 ){        return 1;    }    int a, b, c, d;    a = b = pos1 + 1;    c = d = pos3;    int ans = 1, temp = 0;    while( a <= pos2 ){        while( s2[d] != s1[a] ) d++;        b = a + d - c;        ans *= DFS( a, b, c, d );        temp++;        a = b = b + 1;        c = d = d + 1;    }    return ans * Cn( M, temp );}int main(){    while( scanf( "%d", &M ) && M ){        scanf( "%s%s", s1, s2 );        cout << DFS( 0, strlen( s1 ) - 1, 0, strlen( s2 ) - 1 ) << endl;    }    return 0;}

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