题目链接：https://codeforces.com/contest/1213/
A：
数奇偶个数。
1 /* basic header */ 2 #include <bits/stdc++.h> 3 /* define */ 4 #define ll long long 5 #define dou double 6 #define pb emplace_back 7 #define mp make_pair 8 #define sot(a,b) sort(a+1,a+1+b) 9 #define rep1(i,a,b) for(int i=a;i<=b;++i) 10 #define rep0(i,a,b) for(int i=a;i<b;++i) 11 #define eps 1e-8 12 #define int_inf 0x3f3f3f3f 13 #define ll_inf 0x7f7f7f7f7f7f7f7f 14 #define lson (curpos<<1) 15 #define rson (curpos<<1|1) 16 /* namespace */ 17 using namespace std; 18 /* header end */ 19 20 const int maxn=110; 21 int n,a[maxn]; 22 23 int main() { 24 scanf("%d",&n); 25 int aa=0,bb=0; 26 rep1(i,1,n){ 27 int x; scanf("%d",&x); 28 if (x&1) aa++; else bb++; 29 } 30 printf("%d ",min(aa,bb)); 31 return 0; 32 }
B：
单调栈维护一下即可。
1 /* basic header */ 2 #include <bits/stdc++.h> 3 /* define */ 4 #define ll long long 5 #define dou double 6 #define pb emplace_back 7 #define mp make_pair 8 #define sot(a,b) sort(a+1,a+1+b) 9 #define rep1(i,a,b) for(int i=a;i<=b;++i) 10 #define rep0(i,a,b) for(int i=a;i<b;++i) 11 #define eps 1e-8 12 #define int_inf 0x3f3f3f3f 13 #define ll_inf 0x7f7f7f7f7f7f7f7f 14 #define lson (curpos<<1) 15 #define rson (curpos<<1|1) 16 /* namespace */ 17 using namespace std; 18 /* header end */ 19 20 const int maxn = 150010; 21 int n, a[maxn]; 22 23 int main() { 24 int t; scanf("%d", &t); 25 while (t--) { 26 scanf("%d", &n); 27 rep1(i, 1, n) scanf("%d", &a[i]); 28 stack<int>s; 29 while (!s.empty()) s.pop(); 30 s.push(a[n]); 31 int ans = 0; 32 for (int i = n - 1; i > 0; i--) { 33 if (a[i] > s.top()) ans++; 34 else if (a[i] < s.top()) s.push(a[i]); 35 } 36 printf("%d ", ans); 37 } 38 return 0; 39 }
C：
没什么可说的数学题。
1 /* basic header */ 2 #include <bits/stdc++.h> 3 /* define */ 4 #define ll long long 5 #define dou double 6 #define pb emplace_back 7 #define mp make_pair 8 #define sot(a,b) sort(a+1,a+1+b) 9 #define rep1(i,a,b) for(int i=a;i<=b;++i) 10 #define rep0(i,a,b) for(int i=a;i<b;++i) 11 #define eps 1e-8 12 #define int_inf 0x3f3f3f3f 13 #define ll_inf 0x7f7f7f7f7f7f7f7f 14 #define lson (curpos<<1) 15 #define rson (curpos<<1|1) 16 /* namespace */ 17 using namespace std; 18 /* header end */ 19 20 int main() { 21 int q; scanf("%d", &q); 22 while (q--) { 23 ll n, m; scanf("%lld%lld", &n, &m); 24 if (n < m) { 25 puts("0"); 26 continue; 27 } 28 if (n == m) { 29 printf("%lld ", m % 10); 30 continue; 31 } 32 ll len = n / m, tmp = m, sum = 0; 33 set<int>s; s.clear(); 34 while (!s.count(tmp % 10)) { 35 s.insert(tmp % 10); 36 tmp += m; 37 } 38 tmp -= m; 39 for (auto i : s) sum += i; 40 int round = s.size(); 41 sum = len / round * sum; 42 for (ll i = len / round * tmp + m; i <= n; i += m) { 43 sum += i % 10; 44 } 45 printf("%lld ", sum); 46 } 47 return 0; 48 }
D：
计算每个数对“比它小的能除2到达的数”的贡献，然后每个数取除2次数前k小即可。
1 /* basic header */ 2 #include <bits/stdc++.h> 3 /* define */ 4 #define ll long long 5 #define dou double 6 #define pb emplace_back 7 #define mp make_pair 8 #define sot(a,b) sort(a+1,a+1+b) 9 #define rep1(i,a,b) for(int i=a;i<=b;++i) 10 #define rep0(i,a,b) for(int i=a;i<b;++i) 11 #define eps 1e-8 12 #define int_inf 0x3f3f3f3f 13 #define ll_inf 0x7f7f7f7f7f7f7f7f 14 #define lson (curpos<<1) 15 #define rson (curpos<<1|1) 16 /* namespace */ 17 using namespace std; 18 /* header end */ 19 20 const int maxn = 2e5 + 10; 21 int n, k, ans = int_inf; 22 vector<int>v[maxn]; 23 24 int main() { 25 scanf("%d%d", &n, &k); 26 rep1(i, 1, n) { 27 int x, cnt = 0; scanf("%d", &x); 28 do { 29 v[x].pb(cnt); 30 x >>= 1; 31 cnt++; 32 } while (x); 33 } 34 for (int i = 0; i < maxn; i++) { 35 if ((int)v[i].size() < k) continue; 36 sort(v[i].begin(), v[i].end()); 37 int tot = 0; 38 for (int j = 0; j < k; j++) tot += v[i][j]; 39 ans = min(ans, tot); 40 } 41 printf("%d ", ans); 42 return 0; 43 }
E：
显然不可能无解，只有两种构造方式，验证即可。
1 /* basic header */ 2 #include <bits/stdc++.h> 3 /* define */ 4 #define ll long long 5 #define dou double 6 #define pb emplace_back 7 #define mp make_pair 8 #define sot(a,b) sort(a+1,a+1+b) 9 #define rep1(i,a,b) for(int i=a;i<=b;++i) 10 #define rep0(i,a,b) for(int i=a;i<b;++i) 11 #define eps 1e-8 12 #define int_inf 0x3f3f3f3f 13 #define ll_inf 0x7f7f7f7f7f7f7f7f 14 #define lson (curpos<<1) 15 #define rson (curpos<<1|1) 16 /* namespace */ 17 using namespace std; 18 /* header end */ 19 20 const int maxn = 3e5 + 10; 21 int n, pos[3] = {0, 1, 2}; 22 char s[4], t[4], ans[maxn]; 23 24 int check() { 25 for (int i = 1; i < 3 * n; i++) { 26 if (ans[i - 1] == s[0] && ans[i] == s[1]) return 0; 27 if (ans[i - 1] == t[0] && ans[i] == t[1]) return 0; 28 } 29 return 1; 30 } 31 32 int main() { 33 scanf("%d", &n); 34 scanf("%s", s); 35 scanf("%s", t); 36 puts("YES"); 37 do { 38 int res = 0; 39 for (int k = 0; k < 3; k++) 40 for (int i = 0; i < n; i++) 41 ans[res++] = 'a' + pos[k]; 42 if (check()) { 43 printf("%s ", ans); 44 return 0; 45 } 46 } while (next_permutation(pos, pos + 3)); 47 do { 48 int res = 0; 49 for (int i = 0; i < n; i++) 50 for (int k = 0; k < 3; k++) 51 ans[res++] = 'a' + pos[k]; 52 if (check()) { 53 printf("%s ", ans); 54 return 0; 55 } 56 } while (next_permutation(pos, pos + 3)); 57 return 0; 58 }