Codeforces Edu Round 72 (Rated for Div. 2)

有点难。

题目连接：https://codeforces.com/contest/1217

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A：

想清楚就是一道口算题。

/* basic header */
#include <bits/stdc++.h>
/* define */
#define ll long long
#define dou double
#define pb emplace_back
#define mp make_pair
#define sot(a,b) sort(a+1,a+1+b)
#define rep1(i,a,b) for(int i=a;i<=b;++i)
#define rep0(i,a,b) for(int i=a;i<b;++i)
#define eps 1e-8
#define int_inf 0x3f3f3f3f
#define ll_inf 0x7f7f7f7f7f7f7f7f
#define lson (curpos<<1)
#define rson (curpos<<1|1)
/* namespace */
using namespace std;
/* header end */

int t;

int main() {
    scanf("%d", &t);
    while (t--) {
        int a, b, c; scanf("%d%d%d", &a, &b, &c);
        if (!c) {
            a > b ? puts("1") : puts("0");
        continue;
        }
        printf("%d
", min(c + 1, max((a + c - b + 1) / 2, 0)));
    }
    return 0;
}

B：

贪心。按砍完一刀之后改变头的数量来排序。

/* basic header */
#include <bits/stdc++.h>
/* define */
#define ll long long
#define dou double
#define pb emplace_back
#define mp make_pair
#define sot(a,b) sort(a+1,a+1+b)
#define rep1(i,a,b) for(int i=a;i<=b;++i)
#define rep0(i,a,b) for(int i=a;i<b;++i)
#define eps 1e-8
#define int_inf 0x3f3f3f3f
#define ll_inf 0x7f7f7f7f7f7f7f7f
#define lson (curpos<<1)
#define rson (curpos<<1|1)
/* namespace */
using namespace std;
/* header end */

const int maxn = 200;
pair<int, int>cut[maxn];

int main() {
    int t; scanf("%d", &t);
    while (t--) {
        int n, head, maxx = 0;
        scanf("%d%d", &n, &head);
        rep1(i, 1, n) {
            int a, b; scanf("%d%d", &a, &b);
            maxx = max(a, maxx);
            cut[i].first = a - b, cut[i].second = a;
        }
        head -= maxx;
        sort(cut + 1, cut + 1 + n, greater<pair<int, int>>());
        if (head <= 0) {
            puts("1");
            continue;
        }
        if (cut[1].first <= 0) {
            puts("-1");
            continue;
        }
        int ans = head / cut[1].first + 1;
        if (head % cut[1].first) ans++;
        printf("%d
", ans);
    }
    return 0;
}

C：

巧妙的想法没有，看到时限4s就是O(n^2)暴力开搞。要先预处理出所有连续0的位置。

/* basic header */
#include <bits/stdc++.h>
/* define */
#define ll long long
#define dou double
#define pb emplace_back
#define mp make_pair
#define sot(a,b) sort(a+1,a+1+b)
#define rep1(i,a,b) for(int i=a;i<=b;++i)
#define rep0(i,a,b) for(int i=a;i<b;++i)
#define epre 1e-8
#define int_inf 0x3f3f3f3f
#define ll_inf 0x7f7f7f7f7f7f7f7f
#define lson (curpos<<1)
#define rson (curpos<<1|1)
/* namespace */
using namespace std;
/* header end */

const int maxn = 2e5 + 10;
int t, pre[maxn];
char s[maxn];

int main() {
    scanf("%d", &t);
    while (t--) {
        scanf("%s", s + 1);
        int n = strlen(s + 1);
        ll ans = 0;
        rep1(i, 1, n) {
            if (s[i] == '1') pre[i] = -1;
            else pre[i] = s[i - 1] == '0' ? pre[i - 1] : i;
        }
        rep1(r, 1, n) {
            ll curr = 0;
            for (int l = r; l >= 1; l--)
                if (s[l] == '1') {
                    if (r - l >= 20) break;
                    curr += (1 << (r - l));
                    if (r - l + 1 == curr) ans++;
                } else {
                    if (r + 1 - curr >= pre[l] && r + 1 - curr <= l) ans++;
                    l = pre[l];
                }
        }
        printf("%lld
", ans);
    }
    return 0;
}