2016湖南省省赛G题

                                       G - Parenthesis

 

Bobo has a balanced parenthesis sequence P=p  ₁ p  ₂…p  _n of length n and q questions.

The i-th question is whether P remains balanced after p  _ai and p  _bi  swapped. Note that questions are individual so that they have no affect on others.

Parenthesis sequence S is balanced if and only if:

1.  S is empty;

2.  or there exists  balanced parenthesis sequence A,B such that S=AB;

3.  or there exists  balanced parenthesis sequence S' such that S=(S').

Input

The input contains at most 30 sets. For each set:

The first line contains two integers n,q (2≤n≤10  ⁵,1≤q≤10  ⁵).

The second line contains n characters p  ₁ p  ₂…p  _n.

The i-th of the last q lines contains 2 integers a  _i,b  _i (1≤a  _i,b  _i≤n,a  _i≠b  _i).

OutputFor each question, output "  Yes" if P remains balanced, or "  No" otherwise. Sample Input

4 2
(())
1 3
2 3
2 1
()
1 2

Sample Output

No
Yes
No

题解：就是一个括号配对问题。

下面附上代码：

#include
#include
#include
using namespace std;
char str[100005];
int main()
{
    int n, m;
    while(~scanf("%d%d", &n, &m))
    {
        scanf("%s", str+1);
        int a, b;
        for(int i = 1; i <= m; i++)
        {
            scanf("%d%d", &a, &b);
            if(a>b) swap(a,b);
            if(str[b]=='(' || str[a] == str[b])//这个地方不加前面的str[b]=='('，就会超时。
            {
                printf("Yes\n");
                continue;
            }
            swap(str[a], str[b]);
            int ans = 0;
            for(int i = 1; i <= n; i++)
            {

                if(str[i] == '(') ans++;
                else ans--;
                if(ans < 0) { printf("No\n"); break;}
            }
            if(ans == 0) printf("Yes\n");
            swap(str[a], str[b]);
        }
    }
    return 0;
}