思维——cf1178E

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1000010;
char s[maxn];
bool v[maxn];
int main() {
    scanf("%s", s + 1);
    int n = strlen(s + 1);
    int l, r, ans = 0;
    for (l = 1, r = n - 1; l + 2 <= r; l += 2, r -= 2) {
        for (int j = 0; j < 2; j++)
            for (int k = 0; k < 2; k++) {
                if(s[l + j] == s[r + k]) {
                    v[l + j] = v[r + k] = 1;
                    ans += 2;
                    goto ed;
                }
            }
        ed:
            continue;
    }
    if(ans < n / 2) {
        v[l] = 1;
    }
    for (int i = 1; i <= n; i++) {
        if(v[i] == 1)
            printf("%c", s[i]);
    }
    printf("\n");
}