当然是用栈 和 c字符串耗时更短啦！

#include<iostream>
#include<cstring>
#include<stack>

using namespace std;

const int N = 200010;

char a[N];
char tmp[N];
char stk[N];
int top = -1;
// stack<char> stk;

int main()
{
    scanf("%s", a);
    int n = strlen(a);
    for(int i = 0; i < n; i++)
    {
        bool flag = true;
        if(top != -1 && a[i] == stk[top])
        {
          top--;
          flag = false;
        }

        if(flag) stk[++top] = a[i];

    }

    int cnt = 0;
    while(top != -1)
    {
        tmp[cnt++] = stk[top];
        top--;
    }

    for(int i = cnt - 1; i >= 0; i--)
        printf("%c", tmp[i]);

    return 0;
}