USACO 3.4 American Heritage

/*
ID: jasison2
LANG: C++
TASK: heritage
*/
#include <cstdio>
#include <cstring>
struct Node
{
int l, r;
Node(): l(-1), r(-1) {}
Node(int _l, int _r): l(_l), r(_r) {}
};
Node node[30];
int len, root;
char in[30], pre[30];
void input()
{
scanf("%s", in);
scanf("%s", pre);
len = strlen(in);
root = pre[0]-'A';
for (int i = 0; i < 30; ++ i)
node[i] = Node();
}
void dfs(int il, int ir, int pl, int pr)
{
if (ir-il <= 0) return;
int p = il;
while (in[p] != pre[pl]) p ++;
if (p != il) node[pre[pl]-'A'].l = pre[pl+1]-'A';
if (p != ir) node[pre[pl]-'A'].r = pre[pl+p-il+1]-'A';
dfs(il,p-1,pl+1,pl+p-il);
dfs(p+1,ir,pl+p-il+1,pr);
}
void output(int x)
{
if (x != -1)
{
output(node[x].l);
output(node[x].r);
putchar(x+'A');
}
}
int main()
{
freopen("heritage.in","r",stdin);
freopen("heritage.out","w",stdout);
input();
dfs(0,len-1,0,len-1);
output(root);
printf("\n");
}