目录
7-1 递推公式
7-2 存钱罐
7-3 买东西
7-4 双重子串
7-5 放小球
7-6 最短路径
7-7 统计子序列的个数
7-8 摆放灯笼
7-9 选零食
7-10 1还是2
7-11 最少的门禁数量
7-12 青春猪头之开学了要好好学习
7-13 青春猪头之毕设真头大
7-14 青春猪头之我没学过C语言
7-15 发射小球
7-16 吉利的数字
7-17 买木棒
7-18 切割木棒
7-19 幸运数字
7-20 增加硬币
7-21 三分球
7-22 老虎机
7-23 正交性
7-24 删除不喜欢的数字
7-25 锦标赛
7-1 递推公式
思路:
由于数据范围很大,模拟会TLE,考虑用矩阵运算优化这个类似斐波那契的递归式,再使用快速幂运算优化矩阵乘法,就能在要求的时间范围内通过了。
AC代码:
#include <iostream>
#include <stdio.h>
using namespace std;
typedef long long ll;
const int maxn = 105;
const int P = 1000000007;
struct matrix {
ll m[maxn][maxn];
};
matrix a;
ll n, k;
matrix matrix_multi(matrix a, matrix b) {
matrix ans;
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= n; j++) {
ans.m[i][j] = 0;
for (int k = 1; k <= n; k++) {
ans.m[i][j] = (ans.m[i][j] % P + (a.m[i][k] * b.m[k][j]) % P) % P;
}
}
}
return ans;
}
matrix quick_matrix_pow(matrix a, ll t) {
matrix ans;
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= n; j++) {
if (i == j) ans.m[i][j] = 1;
else ans.m[i][j] = 0;
}
}
while (t > 0) {
if (t & 1)
ans = matrix_multi(a, ans);
a = matrix_multi(a, a);
t >>= 1;
}
return ans;
}
int main() {
n = 3;
cin >> k;
a.m[1][1] = a.m[1][2] = a.m[1][3] = a.m[2][1] = a.m[3][2] = 1;
matrix ans = quick_matrix_pow(a, k - 2);
cout << ((ans.m[1][1] + ans.m[1][2])%P + ans.m[1][3])%P;
return 0;
}
7-2 存钱罐
思路:
二分,等差数列
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
int T,n;
int main() {
cin >> T;
while (T--) {
scanf("%d", &n);
n *= 2;
int l = 1, r = 1e5 + 10;
while (l < r) {
int mid = (l + r) >> 1;
if ((LL)mid * (mid + 1) >= n)r = mid;
else l = mid + 1;
}
printf("%d\n", l);
}
}
7-3 买东西
思路:
贪心,由于每张券可以使物品少花一半的钱,那么给当前最大的物品使用券,一定是最优惠的,所以用一个优先队列(堆)维护当前最贵物品,每次用券,将最贵物品价格除以2,放入堆中,直至券全部用完。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 100010, M = N * 2, mod = 1e9 + 7;
int n, m;
priority_queue<int> q;
int main() {
cin >> n >> m;
for (int i = 0; i < n; i++) {
int x;
cin >> x;
q.push(x);
}
while (m--) {
int x = q.top();
if (x == 0)break;
q.pop();
q.push(x / 2);
}
LL sum = 0;
while (!q.empty()) {
sum += q.top();
q.pop();
}
cout << sum;
}
7-4 双重子串
思路:
dp 我们以dp[i][j] 表示以i为开头的字串和以j为开头的字串最长重复部分的长度。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int N = 5010;
int f[N][N];
int n;
int res;
string s;
int dfs(int x,int y){
if(x > n || y > n)return 0;
if(f[x][y] != -1)return f[x][y];
if(s[x] != s[y])f[x][y] = 0;
else{
f[x][y] = min(y - x, dfs(x+1,y+1) + 1);
}
return f[x][y];
}
int main() {
cin >> n;
cin >> s;
s = ' ' + s;
memset(f,-1,sizeof f);
for(int i=1;i<=n;i++){
for(int j = i + 1;j <= n;j++){
res = max(res,dfs(i,j));
}
}
cout<<res;
return 0;
}
7-5 放小球
题目有一点点问题:
“对于每个i(1≤i≤n),满足编号为i的倍数的箱子内装的小球的总数等于ai%2。”
应改为
“对于每个i(1≤i≤n),满足编号为i的倍数的箱子内装的小球的总数%2等于ai。”
思路:
这道题其实不存在无解的情况,我们总能根据他的条件构造出来一个正解
原因:
我们从n遍历到1, 统计i的倍数放的小球的个数sum,如果sum%2符合要求,那么这个位置就不放小球,如果不符合要求,就在此位置放置小球即可。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 200010, M = N * 2, mod = 1e9 + 7;
int n, m;
int a[N],b[N];
vector<int> res;
int main() {
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = n; i >= 1; i--) {
int sum = 0;
for (int k = 2; i * k <= n; k++)sum += b[i * k];
if (sum % 2 == 0) {
if (a[i]) {
res.push_back(i);
b[i] = 1;
}
}
else {
if(!a[i]) {
res.push_back(i);
b[i] = 1;
}
}
}
cout << res.size() << endl;
for (int i = 0; i < res.size(); i++) {
cout << res[i] << " ";
}
}
7-6 最短路径
思路:
此题点与点之间的距离是1,可以考虑bfs,只不过每次都要走3步,因此我们的st数组和d数组都应该是模3意义下的。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int N = 1e5 + 10;
int n, m, p;
int h[N], e[N], ne[N], idx;
void add(int a, int b) {
e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
int a, b, x, y;
bool st[N][3];
int d[N][3];
int main() {
scanf("%d%d", &n, &m);
memset(h, -1, sizeof h);
memset(st, 0, sizeof st);
memset(d, -1, sizeof d);
while (m--) {
scanf("%d%d", &a, &b);
add(a, b);
}
scanf("%d%d", &x, &y);
st[x][0] = true;
d[x][0] = 0;
queue<PII> q;
q.push({ x,d[x][0] });
while (q.size()) {
PII t = q.front();
q.pop();
int idx = t.first;
int dist = t.second + 1;
int fg = dist % 3;
for (int i = h[idx]; i != -1; i = ne[i]) {
int j = e[i];
if (st[j][fg])continue;
st[j][fg] = true;
d[j][fg] = dist;
if (j == y && fg == 0) {
cout << d[j][0] / 3;
return 0;
}
q.push({j,d[j][fg]});
}
}
cout << "-1";
return 0;
}
7-7 统计子序列的个数
思路:
dp ,我们用dp[i][j] 来表示a数组中i位置之前,b数组中j位置之前的相同子序列的个数。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 2010, M = 1e6+10, mod = 1e9 + 7;
int n, m;
LL a[N], b[N];
LL f[N][N];
int main()
{
cin >> n >> m;
for (int i = 1; i <= n; i++)cin >> a[i];
for (int j = 1; j <= m; j++)cin >> b[j];
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= m; j++) {
f[i][j] = (f[i][j - 1] + f[i - 1][j] - f[i - 1][j - 1]) % mod;
if (a[i] == b[j])f[i][j] += (f[i - 1][j - 1] + 1) % mod;
if (f[i][j] < 0)f[i][j] += mod;
if (f[i][j] > mod)f[i][j] %= mod;
}
}
cout << f[n][m] + 1;
return 0;
}
7-8 摆放灯笼
思路:
模拟即可。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e4+10, M = 1e6+10, mod = 1e9 + 7;
int n, m;
char g[N][N];
int tran[N][N], dir[N][N];
int main()
{
cin >> n >> m;
for (int i = 0; i < n; i++)cin >> g[i];
for (int i = 0; i < n; i++)
for (int j = 0; j < m; j++) {
if (j - 1 < 0 || g[i][j - 1] == '#') {
int res = 0;
int k = j;
while (g[i][k] == '.')k++,res++;
tran[i][j] = res;
}
else tran[i][j] = tran[i][j - 1];
}
for (int j = 0; j < m; j++)
for (int i = 0; i < n; i++) {
if (i - 1 < 0 || g[i - 1][j] == '#') {
int res = 0;
int k = i;
while (g[k][j] == '.')k++, res++;
dir[i][j] = res;
}
else dir[i][j] = dir[i - 1][j];
}
int res = 0;
for(int i=0;i<n;i++)
for (int j = 0; j < m; j++) {
if (g[i][j] == '#')continue;
else res = max(res, tran[i][j] + dir[i][j] - 1);
}
cout << res;
return 0;
}
7-9 选零食
思路:
模拟即可,枚举取几个零食,放回几个零食
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int N = 55;
int n, m;
int a[N];
int res;
int main() {
cin >> n >> m;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
int buy = min(n, m);
for (int i = 1; i <= buy; i++) {
int put = m - i;
for (int jl = 0; jl <= i; jl++) {
int sum = 0;
priority_queue<int, vector<int>, greater<int>> q;
int jr = n - i + jl + 1;
for (int j = 1; j <= jl; j++) {
q.push(a[j]);
sum += a[j];
}
for (int j = jr; j <= n; j++) {
q.push(a[j]);
sum += a[j];
}
int x = put;
while (!q.empty() && q.top() < 0 && x > 0) {
sum -= q.top();
q.pop();
x--;
}
res = max(res, sum);
}
}
cout << res;
return 0;
}
7-10 1还是2
思路:
考虑一件事情:如果我们得到了这样的线索:“1 和 2 、2 和3 、3和4”,我们只要知道1,就可以把2、3、4推断出来了。因此,判断线索将哪些卡片联系起来,成为了一组,每组中我们只要知道其中一个,其余的就都可以推断出来了。
使用并查集
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e5+10, M = 1e6+10, mod = 1e9 + 7;
int n, m;
int p[N];
int find(int x) {
if (x != p[x])p[x] = find(p[x]);
return p[x];
}
int main()
{
cin >> n >> m;
for (int i = 1; i <= n; i++)p[i] = i;
for (int i = 0; i < m; i++) {
int a, b, c;
cin >> a >> b >> c;
a = find(a), b = find(b);
if (a != b)p[a] = b;
}
set<int> st;
for (int i = 1; i <= n; i++) {
p[i] = find(p[i]);
st.insert(p[i]);
}
cout << st.size();
return 0;
}
7-11 最少的门禁数量
思路:
典型差分,将所有大门的信息差分,求一遍前缀和,当前和等于门的数量的钥匙即满足要求
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e5+10, M = 1e6+10, mod = 1e9 + 7;
int n, m;
int chafen[N];
int main()
{
cin >> n >> m;
int men = m;
while (m--) {
int l, r;
cin >> l >> r;
chafen[l] += 1;
chafen[r + 1] -= 1;
}
int res = 0;
for (int i = 1; i <= n; i++) {
chafen[i] += chafen[i - 1];
if (chafen[i] == men)res++;
}
cout << res;
return 0;
}
7-12 青春猪头之开学了要好好学习
思路:
由于我们可以进行无限次操作,所以如果负数个数为偶数的话,我们总能将其全变为正数。
但如果负数个数为奇数,我们不管怎么翻转,最后都会留下一个负数,那我们一定会留最小的那个负数。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e5+10, M = 1e6+10, mod = 1e9 + 7;
int n, m;
vector<LL> a;
int main()
{
LL res = 0;
cin >> n;
for (int i = 0; i < n; i++){
LL x;
cin >> x;
if (x < 0)m++;
a.push_back(x);
}
if (m % 2 == 0) {
for (int i = 0; i < n; i++)res += abs(a[i]);
}
else {
for (int i = 0; i < n; i++) {
a[i] = abs(a[i]);
}
sort(a.begin(), a.end());
for (int i = 0; i < n; i++) {
if (!i)res += (-1) * a[i];
else res += a[i];
}
}
cout << res;
return 0;
}
7-13 青春猪头之毕设真头大
思路:(双指针)
枚举每一个0序列区间,判断将其全变成1,和两边的1相连长度是多少,更新最大值
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e5+10, M = 1e6+10, mod = 1e9 + 7;
int n, m;
string s;
int main()
{
cin >> n >> m;
getchar();
cin >> s;
int l = 0, r = 0, cnt = 0, num = 0;
while (r < n) {
if (s[r] == '0') {
while (s[r] == '0'&& r < n - 1)r++;
cnt++;
}
while (cnt > m) {
while (s[l] == '1' && l < n - 1)l++;
while (s[l] != '1' && l < n - 1)l++;
cnt--;
}
num = max(num, r - l + 1);
r++;
}
cout << num;
return 0;
}
7-14 青春猪头之我没学过C语言
思路:
从1到n的异或值有规律,可以打表观察。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 1e5+10, M = 1e6+10, mod = 1e9 + 7;
int n;
LL x, y;
LL fun(LL x) {
if (x % 4 == 1)return 1;
else if (x % 4 == 2)return x + 1;
else if (x % 4 == 3)return 0;
else return x;
}
int main()
{
cin >> x >> y;
LL res = fun(max((LL)0,x - 1)) ^ fun(y);
cout << res;
return 0;
}
7-15 发射小球
思路:
推公式即可
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
int main()
{
double sx, sy, gx, gy;
double k = 0;
scanf("%lf%lf%lf%lf", &sx, &sy, &gx, &gy);
gy = -gy;
printf("%.10lf", sx - (double)(sx-gx)/(sy-gy)*sy);
return 0;
}
7-16 吉利的数字
思路:
只需要判断所有出现过的数字能组成的3位数是否是8的倍数即可。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
typedef long long LL;
const int N = 2e5 + 10;
int st[11];
char s[N];
int main()
{
cin >> s;
if (strlen(s) == 1) {
if ((s[0] - '0') % 8 == 0)cout << "Yes";
else cout << "No";
return 0;
}
if (strlen(s) == 2) {
int t = 0, x = 0;
t += s[0] - '0';
t *= 10;
t += s[1] - '0';
x += s[1] - '0';
x *= 10;
x += s[0] - '0';
if (t % 8 == 0 || x % 8 == 0)cout << "Yes";
else cout << "No";
return 0;
}
for (int i = 0; i < strlen(s); i++) {
int x = s[i] - '0';
st[x]++;
}
for (int i = 1; i < 10; i++)
for (int j = 1; j < 10; j++)
for (int k = 1; k < 10; k++) {
int t = 0;
bool fg1 = false, fg2 = false, fg3 = false;
if (st[i]) {
t += i;
st[i]--;
fg1 = true;
}else continue;
if (st[j]) {
t *= 10;
t += j;
st[j]--;
fg2 = true;
}else{
st[i]++;
continue;
}
if (st[k]) {
t *= 10;
t += k;
st[k]--;
fg3 = true;
}else{
st[i]++;
st[j]++;
continue;
}
if (fg1)st[i]++;
if (fg2)st[j]++;
if (fg3)st[k]++;
if (t && t % 8 == 0) {
cout << "Yes";
return 0;
}
}
cout << "No";
return 0;
}
7-17 买木棒
思路:
贪心,肯定要购买长度为n+1的木棒,考虑其能最多分割成多少个小的木棒,其余的购买。
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
LL S, P;
int main()
{
cin >> S;
P = S;
S++;
int i = 1;
while (S >= i) {
S -= i;
i++;
}
cout << P - i + 2;
return 0;
}
7-18 切割木棒
思路:
长度为n的木棒,找11个切割位置:
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 210;
LL f[N][13];
int main()
{
for (int i = 0; i < N; i++) {
for (int j = 0; j <= i && j < 13; j++) {
if (j == 0)f[i][j] = (LL)1;
else f[i][j] = f[i - 1][j - 1] + f[i - 1][j];
}
}
int n; cin >> n;
cout << f[n - 1][11];
return 0;
}
7-19 幸运数字
思路:
预处理:线性筛法求素数,求其前缀和
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 1e5+10;
int prime[N],cnt;
int s[N];
bool st[N];
void get_prime(int n){
for(int i=2;i<=n;i++){
if(!st[i]){
prime[cnt++]=i;
}
for(int j=0;prime[j]<=n/i;j++){
st[prime[j]*i]=true;
if(i%prime[j]==0)break;
}
}
}
int main()
{
get_prime(N-1);
for(int i=2;i<N;i++){
if(!st[i] && !st[(i+1)/2] && i%2==1)s[i]=1;
}
for(int i=2;i<N;i++)s[i]+=s[i-1];
int q;
cin>>q;
while(q--){
int l,r;
scanf("%d%d",&l,&r);
printf("%d\n",s[r]-s[l-1]);
}
return 0;
}
7-20 增加硬币
思路:
期望DP
来源: AT_abc184_d [ABC184D] increment of coins - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 110;
double f[N][N][N];
int d = 0;
double dp(int a, int b, int c) {
if (f[a][b][c] >= 0)return f[a][b][c];
f[a][b][c] = 0;
if (a >= 100 || b >= 100 || c >= 100)return f[a][b][c];
int sum = a + b + c;
f[a][b][c] += (dp(a + 1, b, c) + 1) * (1.0 * a) / sum;
f[a][b][c] += (dp(a, b + 1, c) + 1) * (1.0 * b) / sum;
f[a][b][c] += (dp(a, b, c + 1) + 1) * (1.0 * c) / sum;
return f[a][b][c];
}
int main()
{
int a, b, c;
scanf("%d%d%d", &a, &b, &c);
memset(f, -1, sizeof f);
printf("%.9lf", dp(a, b, c));
return 0;
}
7-21 三分球
自己写吧
7-22 老虎机
同上
7-23 正交性
模拟
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 100010;
int a[N];
int main()
{
int n;
scanf("%d",&n);
for(int i=1;i<=n;i++)scanf("%d",&a[i]);
for(int i=1;i<=n;i++){
int x;
scanf("%d",&x);
a[i]*=x;
a[i]+=a[i-1];
}
if(a[n]==0)cout<<"Yes";
else cout<<"No";
return 0;
}
7-24 删除不喜欢的数字
模拟
代码:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 100010;
int a[N];
int main()
{
int n,m;
scanf("%d%d",&n,&m);
for(int i=1;i<=n;i++){
int x;
scanf("%d",&x);
if(x==m)continue;
printf("%d ",x);
}
return 0;
}
7-25 锦标赛
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 20;
int n;
int res1,res2;
int d1,d2;
int main()
{
cin>>n;
n = 1<<n;
for(int i=1;i<=n;i++){
int x;
scanf("%d",&x);
if(i<=n/2){
if(res1<x){
res1=x;
d1=i;
}
}else{
if(res2<x){
res2=x;
d2=i;
}
}
}
if(res1>res2)cout<<d2;
else cout<<d1;
//if(a[n]==0)cout<<"Yes";
//else cout<<"No";
return 0;
}