题目传送门
1 /*
2 水题：遍历一边先找AB，再BA，再遍历一边先找BA，再AB，两种情况满足一种就YES
3 */
4 #include <cstdio>
5 #include <iostream>
6 #include <cstring>
7 #include <cmath>
8 #include <algorithm>
9 #include <vector>
10 #include <map>
11 #include <queue>
12 #include <set>
13 #include <string>
14 #include <stack>
15 using namespace std;
16
17 typedef long long ll;
18 const int MAXN = 1e5 + 10;
19 const int INF = 0x3f3f3f3f;
20 char s[MAXN];
21
22 int main(void) //Codeforces Round #306 (Div. 2) A. Two Substrings
23 {
24 while (scanf ("%s", s) == 1)
25 {
26 bool ok1 = false, ok2 = false, ok3 = false, ok4 = false;
27 int len = strlen (s); int p = 0;
28 for (int i=0; i<len; ++i)
29 {
30 if (i < len - 1 && !ok1 && s[i] == 'A' && s[i+1] == 'B')
31 {
32 ok1 = true; i++;
33 }
34 else if (i < len - 1 && ok1 && s[i] == 'B' && s[i+1] == 'A')
35 {
36 ok2 = true; break;
37 }
38 }
39 for (int i=0; i<len; ++i)
40 {
41 if (i < len - 1 && !ok3 && s[i] == 'B' && s[i+1] == 'A')
42 {
43 ok3 = true; i++;
44 }
45 else if (i < len - 1 && ok3 && s[i] == 'A' && s[i+1] == 'B')
46 {
47 ok4 = true; break;
48 }
49 }
50
51 if ((ok1 && ok2) || (ok3 && ok4)) puts ("YES");
52 else puts ("NO");
53 }
54
55
56 return 0;
57 }
58
59 /*
60 ABA
61 BACFAB
62 AXBYBXA
63 */