cf D. Alternating Current

http://codeforces.com/contest/344/problem/D

 #include <cstdio>
 #include <cstring>
 #include <stack>
 #include <algorithm>
 using namespace std;

 char str[];
 int main()
 {
     stack<char>q;
     scanf("%s",str);
     int k=strlen(str);
     for(int i=; i<k; i++)
     {
         if(q.empty())
         {
             q.push(str[i]);
         }
         else if(str[i]==q.top())
         {
             q.pop();
         }
         else if(str[i]!=q.top())
         {
             q.push(str[i]);
         }
     }
     if(!q.empty()) printf("No\n");
     else printf("Yes\n");
     return ;
 }