【CF】556D A Lot of Games

构建trie树，可以得到4类结点：必胜点，必负点，完全主宰点（可胜可负），完全无法主宰点（无法控制最终胜负）。递归到叶子结点，即为必胜点，回溯分情况讨论。注意叶子结点使用属性n来控制，n表示当前结点的儿子结点的数目，叶子结点没有儿子。

 /* 456D */
 #include <iostream>
 #include <string>
 #include <map>
 #include <queue>
 #include <set>
 #include <stack>
 #include <vector>
 #include <deque>
 #include <algorithm>
 #include <cstdio>
 #include <cmath>
 #include <ctime>
 #include <cstring>
 #include <climits>
 #include <cctype>
 #include <cassert>
 using namespace std;

 typedef struct trie_t {
     int n;
     int next[];
     trie_t() {
         n = ;
         memset(next, , sizeof(next));
     }
 } trie_t;

 const int maxn = 1e5+;

 trie_t T[maxn];
 int L = ;
 char s[maxn];
 int n, m;

 int newTrie() {
     return ++L;
 }

 void create(char *s, int rt) {
     int p = rt;
     int i = , id;

     while (s[i]) {
         id = s[i] - 'a';
         if (T[p].next[id] == ) {
             T[p].next[id] = ++L;
             ++T[p].n;
         }
         p = T[p].next[id];
         ++i;
     }
 }

 int dfs(int rt) {
     int i;
     int st = ;

     if (T[rt].n == )
         return ;

     for (i=; i<; ++i) {
         if (T[rt].next[i])
             st |= dfs(T[rt].next[i]);
     }
     switch(st) {
     case :
         return ;
     case :
         return ;
     case :
         return ;
     case :
         return ;
     default:
         return ;
     }
 }

 int main() {
     int i, j, k;

     #ifndef ONLINE_JUDGE
         freopen("data.in", "r", stdin);
         freopen("data.out", "w", stdout);
     #endif

     scanf("%d %d", &n, &m);
     // build trie
     for (i=; i<n; ++i) {
         scanf("%s", s);
         create(s, );
     }
     // get the key point
     int st = ;
     for (i=; i<; ++i) {
         if (T[].next[i])
             st |= dfs(T[].next[i]);
     }

     if (st == ) {
         puts("First");
         return ;
     } else if (st == ) {
         puts("Second");
         return ;
     }
     if (st == ) {
         puts("Second");
         return ;
     }
     if (m & ) {
         puts("First");
     } else {
         puts("Second");
     }

     #ifndef ONLINE_JUDGE
         printf("time = %d.\n", (int)clock());
     #endif

     return ;
 }