Codeforces Round #706 (Div. 2)（补题）

A.

    #include<bits/stdc++.h>
    using namespace std;
    int main () {
        int t;
        cin >> t;
        while(t--) {
            int n, k;
            cin >> n >> k;
            string s;
            cin >> s;
            if(k == 0) {
                cout << "YES" << endl;
                continue;
            }
            int gap = (n & 1) ?  n / 2 : n / 2 - 1;
            if(gap < k) {
                cout << "NO" << endl;
                continue;
            }
            bool isok = 0;
            for(int i = 0; i < k; ++i) {
                if(s[i] != s[n - i - 1]) {
                    isok = 1;
                    break;
                }
            }
            if(isok) {
                cout << "NO" << endl;
            } else {
                cout << "YES" << endl;
            }
        }
    }

B

    #include<bits/stdc++.h>
    using namespace std;
    typedef long long ll;
    int main () {
        int t;
        cin >> t;
        while(t--) {
            ll n, k;
            cin >> n >> k;
            std::vector<ll> v(n);
            ll a[100010];
            std::map<ll, int> mp;
            for(int i = 0; i <= 100001; ++i) {
                a[i] = 0;
            }
            ll maxx = -1;
            for(int i = 0; i < n; ++i) {
                cin >> v[i];
                if(v[i] <= 100001) a[v[i]]++;
                mp[v[i]] = 1;
                maxx = max(maxx, v[i]);
            }
            ll mexx = -1;
            for(int i = 0; i <= 100001; ++i) {
                if(a[i] == 0) {
                    mexx = i;
                    break;
                }
            }
            ll keyy = ll((double(mexx) + double(maxx)) / 2.0 + 0.5);
            if(k == 0 || mp[keyy] == 1) {
                cout << n << endl;
            } else if(keyy > maxx) {
                cout << n + k << endl;
            } else {
                cout << n + 1 << endl;
            }
        }
    }

C

    #include<bits/stdc++.h>
    using namespace std;
    typedef long long ll;
    int main () {
        ios::sync_with_stdio(false);
        int t;
        cin >> t;
        cout.precision(15);
        while(t--) {
            int n;
            cin >> n;
            std::vector<ll> vx;
            std::vector<ll> vy; 
            for(int i = 0; i < 2 * n; ++i) {
                ll x, y;
                cin >> x >> y;
                if(x == 0) vy.push_back(abs(y));
                else vx.push_back(abs(x));           
            } 
            sort(vx.begin(), vx.end());
            sort(vy.begin(), vy.end());
            double ans = 0.0;
            for(int i = 0; i < n; ++i) {
                ans += sqrt(vx[i] * vx[i] + vy[i] * vy[i]);  
            }
            cout << ans << endl;
        }
    }

注意不要用double 容易t