#include <cstdio>

char a[300010], k;

int main(){
    char c;
    while((c = getchar()) != EOF){
        if(k == 0 || a[k - 1] != c)
            a[k++] = c;
        else
            k--;
    }
    for(int i = 0; i < k; i++)
        putchar(a[i]);
    return 0;
}

This one gives RE(AV) on the 6th test. I tried it 4 times with different sizes of array "a" (200000, 200010, 300010, 2000000) and each time got this error. I found it very strange, could you help me please?

simple, int k(0);