http://codeforces.com/contest/612/problem/C 题意：其实就是栈的基本操作。。水题。
代码实现
1/* ***********************************************
2Author :111qqz
3Created Time :2015年12月25日 星期五 22时58分50秒
4File Name :code/cf/edu4/C.cpp
5************************************************ */
6
7#include <cstdio>
8#include <cstring>
9#include <iostream>
10#include <algorithm>
11#include <vector>
12#include <queue>
13#include <set>
14#include <map>
15#include <string>
16#include <cmath>
17#include <cstdlib>
18#include <ctime>
19#define fst first
20#define sec second
21#define lson l,m,rt<<1
22#define rson m+1,r,rt<<1|1
23#define ms(a,x) memset(a,x,sizeof(a))
24typedef long long LL;
25#define pi pair < int ,int >
26#define MP make_pair
27
28using namespace std;
29const double eps = 1E-8;
30const int dx4[4]={1,0,0,-1};
31const int dy4[4]={0,-1,1,0};
32const int inf = 0x3f3f3f3f;
33const int N=1E6+7;
34int len;
35char st[N];
36int cost = 0 ;
37
38char a[N];
39int n ;
40
41
42int which(char ch)
43{
44 if (ch=='<'||ch=='{'||ch=='('||ch=='[') return 1;
45 return 2;
46}
47int kin(char ch)
48{
49 if (ch=='{'||ch=='}') return 1;
50 if (ch=='['||ch==']') return 2;
51 if (ch=='<'||ch=='>') return 3;
52 if (ch=='('||ch==')') return 4;
53}
54bool ok(char x,char y)
55{
56 int res = 0 ;
57 if(x=='<'||x=='{'||x=='['||x=='(') res++;
58 if (y=='>'||y=='}'||y==']'||y==')') res++;
59 if (res==2)
60 {
61 if (kin(x)!=kin(y)) cost++;
62// cout<<"x:"<<x<<" y:"<<y<<endl;
63 return true;
64 }
65 return false;
66}
67
68int main()
69{
70 #ifndef ONLINE_JUDGE
71 freopen("code/in.txt","r",stdin);
72 #endif
73 cin>>st;
74 len = strlen(st);
75 int head = -1;
76 int ans = 0 ;
77
78 for ( int i = 0 ; i < len ; i++)
79 {
80 if (head==-1)
81 {
82 head++;
83 a[head] = st[i];
84 if (which(st[i])==2)
85 {
86 puts("Impossible");
87 return 0;
88 }
89
90 continue;
91 }
92 if (ok(a[head],st[i]))
93 {
94 head--;
95 }
96 else
97 {
98 head++;
99 a[head] = st[i];
100 }
101
102// cout<<"head:"<<head<<endl;
103 }
104// cout<<"head:"<<head<<endl;
105 if (head!=-1)
106 {
107 puts("Impossible");
108 }
109 else
110 {
111 cout<<cost<<endl;
112 }
113
114 #ifndef ONLINE_JUDGE
115 fclose(stdin);
116 #endif
117 return 0;
118}