【离散化 前缀和 二分 滑动窗口】P2862 [USACO06JAN] Corral the Cows G|普及+

本文涉及的基础知识点

C++二分查找
C++算法：滑动窗口及双指针总结
C++算法：前缀和、前缀乘积、前缀异或的原理、源码及测试用例 包括课程视频

[USACO06JAN] Corral the Cows G

题目描述

Farmer John wishes to build a corral for his cows. Being finicky beasts, they demand that the corral be square and that the corral contain at least C (1 <= C <= 500) clover fields for afternoon treats. The corral’s edges must be parallel to the X,Y axes.

FJ’s land contains a total of N (C <= N <= 500) clover fields, each a block of size 1 x 1 and located at with its lower left corner at integer X and Y coordinates each in the range 1…10,000. Sometimes more than one clover field grows at the same location; such a field would have its location appear twice (or more) in the input. A corral surrounds a clover field if the field is entirely located inside the corral’s borders.

Help FJ by telling him the side length of the smallest square containing C clover fields.

Farmer John 希望为他的奶牛们建立一个畜栏。

这些挑剔的奶牛要求畜栏必须是正方形的，而且至少要包含 $C$ 片三叶草，来当做它们的下午茶。

约翰的土地里一共包含 $N$ 片三叶草，每单位三叶草位于一个 $1\times 1$ 的土地区域内。坐标都为整数，范围在 $1$ 到 $10000$ 以内。

多片的三叶草可能会位于同一个 $1\times 1$ 的区域内，所以在输入内，同一个区域坐标可能出现多次。

请你帮 Farmer John 计算一下，在能包含至少 $C$ 片三叶草的情况下，畜栏的最小边长是多少。

$1\le C\le N\le 500$。

输入格式

Line 1: Two space-separated integers: C and N

Lines 2…N+1: Each line contains two space-separated integers that are the X,Y coordinates of a clover field.

输出格式

Line 1: A single line with a single integer that is length of one edge of the minimum size square that contains at least C clover fields.

样例 #1

样例输入 #1

3 4
1 2
2 1
4 1
5 2

样例输出 #1

4

提示

Explanation of the sample:

|*   *
| * *
+------

Below is one 4x4 solution (C’s show most of the corral’s area); many others exist.

|CCCC
|CCCC
|*CCC*
|C*C*
+------

离散化+二维前缀和+二分查找+滑动窗口

令M是三叶草的最大坐标，本题是10000。
** 性质一**：本题坐标无限大，假定右下是正方向，如果正方形左上角是(l,t)则正方向的第l列和t行，必定有三叶草，否则l++或t++。
离散后，二维前缀和O(NN)。
二分首端：任意边长mid的正方向包括的三叶草数量大于等于C。
**时间复杂度：O(NNlogMlogn)可能超时。
二分边长：O(logM)
枚举左上角：O(NN)
二分右下角：O(logn)
用滑动窗口，处理右下角。时间复杂度：O(1)。故总时间复杂度：O(nnlogM)

代码

核心代码

#include 
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#include 
using namespace std;

template<class T1, class T2>
std::istream& operator >> (std::istream& in, pair<T1, T2>& pr) {
	in >> pr.first >> pr.second;
	return in;
}

template<class T1, class T2, class T3 >
std::istream& operator >> (std::istream& in, tuple<T1, T2, T3>& t) {
	in >> get<0>(t) >> get<1>(t) >> get<2>(t) ;
	return in;
}

template<class T1, class T2, class T3, class T4 >
std::istream& operator >> (std::istream& in, tuple<T1, T2, T3, T4>& t) {
	in >> get<0>(t) >> get<1>(t) >> get<2>(t) >> get<3>(t);
	return in;
}

template<class T = int>
vector<T> Read() {
	int n;
	scanf("%d", &n);
	vector<T> ret(n);
	for(int i=0;i < n ;i++) {
		cin >> ret[i];
	}
	return ret;
}

template<class T = int>
vector<T> Read(int n) {
	vector<T> ret(n);
	for (int i = 0; i < n; i++) {
		cin >> ret[i];
	}
	return ret;
}

template<int N = 12 * 1'000'000>
class COutBuff
{
public:
	COutBuff() {
		m_p = puffer;
	}
	template<class T>
	void write(T x) {
		int num[28], sp = 0;
		if (x < 0)
			*m_p++ = '-', x = -x;

		if (!x)
			*m_p++ = 48;

		while (x)
			num[++sp] = x % 10, x /= 10;

		while (sp)
			*m_p++ = num[sp--] + 48;
	}
	inline void write(char ch)
	{
		*m_p++ = ch;
	}
	inline void ToFile() {
		fwrite(puffer, 1, m_p - puffer, stdout);
	}
private:
	char  puffer[N], * m_p;
};

template<int N = 12 * 1'000'000>
class CInBuff
{
public:
	inline CInBuff() {
		fread(buffer, 1, N, stdin);
	}
	inline int Read() {
		int x(0), f(0);
		while (!isdigit(*S))
			f |= (*S++ == '-');
		while (isdigit(*S))
			x = (x << 1) + (x << 3) + (*S++ ^ 48);
		return f ? -x : x;
	}
private:
	char buffer[N], * S = buffer;
};

template<class TSave, class TRecord >
class CRangUpdateLineTree
{
protected:
	virtual void OnQuery(const TSave& save, const int& iSaveLeft, const int& iSaveRight) = 0;
	virtual void OnUpdate(TSave& save, const int& iSaveLeft, const int& iSaveRight, const TRecord& update) = 0;
	virtual void OnUpdateParent(TSave& par, const TSave& left, const TSave& r, const int& iSaveLeft, const int& iSaveRight) = 0;
	virtual void OnUpdateRecord(TRecord& old, const TRecord& newRecord) = 0;
};

template<class TSave, class TRecord >
class CVectorRangeUpdateLineTree : public CRangUpdateLineTree<TSave, TRecord>
{
public:
	CVectorRangeUpdateLineTree(int iEleSize, TSave tDefault, TRecord tRecordNull) :m_iEleSize(iEleSize)
		, m_save(iEleSize * 4, tDefault), m_record(iEleSize * 4, tRecordNull) {
		m_recordNull = tRecordNull;
	}
	void Update(int iLeftIndex, int iRightIndex, TRecord value)
	{
		Update(1, 0, m_iEleSize - 1, iLeftIndex, iRightIndex, value);
	}
	void Query(int leftIndex, int rightIndex) {
		Query(1, 0, m_iEleSize - 1, leftIndex, rightIndex);
	}
	//void Init() {
	//	Init(1, 0, m_iEleSize - 1);
	//}
	TSave QueryAll() {
		return m_save[1];
	}
	void swap(CVectorRangeUpdateLineTree<TSave, TRecord>& other) {
		m_save.swap(other.m_save);
		m_record.swap(other.m_record);
		std::swap(m_recordNull, other.m_recordNull);
		assert(m_iEleSize == other.m_iEleSize);
	}
protected:
	//void Init(int iNodeNO, int iSaveLeft, int iSaveRight)
	//{
	//	if (iSaveLeft == iSaveRight) {
	//		this->OnInit(m_save[iNodeNO], iSaveLeft);
	//		return;
	//	}
	//	const int mid = iSaveLeft + (iSaveRight - iSaveLeft) / 2;
	//	Init(iNodeNO * 2, iSaveLeft, mid);
	//	Init(iNodeNO * 2 + 1, mid + 1, iSaveRight);
	//	this->OnUpdateParent(m_save[iNodeNO], m_save[iNodeNO * 2], m_save[iNodeNO * 2 + 1], iSaveLeft, iSaveRight);
	//}
	void Query(int iNodeNO, int iSaveLeft, int iSaveRight, int iQueryLeft, int iQueryRight) {
		if ((iSaveLeft >= iQueryLeft) && (iSaveRight <= iQueryRight)) {
			this->OnQuery(m_save[iNodeNO], iSaveLeft, iSaveRight);
			return;
		}
		if (iSaveLeft == iSaveRight) {//没有子节点
			return;
		}
		Fresh(iNodeNO, iSaveLeft, iSaveRight);
		const int mid = iSaveLeft + (iSaveRight - iSaveLeft) / 2;
		if (mid >= iQueryLeft) {
			Query(iNodeNO * 2, iSaveLeft, mid, iQueryLeft, iQueryRight);
		}
		if (mid + 1 <= iQueryRight) {
			Query(iNodeNO * 2 + 1, mid + 1, iSaveRight, iQueryLeft, iQueryRight);
		}
	}
	void Update(int iNode, int iSaveLeft, int iSaveRight, int iOpeLeft, int iOpeRight, TRecord value)
	{
		if ((iOpeLeft <= iSaveLeft) && (iOpeRight >= iSaveRight))
		{
			this->OnUpdate(m_save[iNode], iSaveLeft, iSaveRight, value);
			this->OnUpdateRecord(m_record[iNode], value);
			return;
		}
		if (iSaveLeft == iSaveRight) {
			return;//没有子节点
		}
		Fresh(iNode, iSaveLeft, iSaveRight);
		const int iMid = iSaveLeft + (iSaveRight - iSaveLeft) / 2;
		if (iMid >= iOpeLeft)
		{
			Update(iNode * 2, iSaveLeft, iMid, iOpeLeft, iOpeRight, value);
		}
		if (iMid + 1 <= iOpeRight)
		{
			Update(iNode * 2 + 1, iMid + 1, iSaveRight, iOpeLeft, iOpeRight, value);
		}
		// 如果有后代，至少两个后代
		this->OnUpdateParent(m_save[iNode], m_save[iNode * 2], m_save[iNode * 2 + 1], iSaveLeft, iSaveRight);
	}
	void Fresh(int iNode, int iDataLeft, int iDataRight)
	{
		if (m_recordNull == m_record[iNode])
		{
			return;
		}
		const int iMid = iDataLeft + (iDataRight - iDataLeft) / 2;
		Update(iNode * 2, iDataLeft, iMid, iDataLeft, iMid, m_record[iNode]);
		Update(iNode * 2 + 1, iMid + 1, iDataRight, iMid + 1, iDataRight, m_record[iNode]);
		m_record[iNode] = m_recordNull;
	}
	vector<TSave> m_save;
	vector<TRecord> m_record;
	TRecord m_recordNull;
	const int m_iEleSize;
};

template<class T = int>
class CDiscretize //离散化
{
public:
	CDiscretize(vector<T> nums)
	{
		sort(nums.begin(), nums.end());
		nums.erase(std::unique(nums.begin(), nums.end()), nums.end());
		m_nums = nums;
		for (int i = 0; i < nums.size(); i++)
		{
			m_mValueToIndex[nums[i]] = i;
		}
	}
	int operator[](const T value)const
	{
		auto it = m_mValueToIndex.find(value);
		if (m_mValueToIndex.end() == it)
		{
			return -1;
		}
		return it->second;
	}
	int size()const
	{
		return m_mValueToIndex.size();
	}
	vector<T> m_nums;
protected:
	unordered_map<T, int> m_mValueToIndex;
};

template<class T = int>
class CPreSum2 {
public:
	template<class _Pr>
	CPreSum2(int rowCnt, int colCount, _Pr pr) :m_iRowCnt(rowCnt), m_iColCnt(colCount) {
		m_vSum.assign(rowCnt + 1, vector<int>(colCount + 1));
		for (int r = 0; r < rowCnt; r++) {
			for (int c = 0; c < colCount; c++) {
				m_vSum[r + 1][c + 1] = m_vSum[r][c + 1] + m_vSum[r + 1][c] - m_vSum[r][c] + pr(r, c);
			}
		}
	}
	T Get(int left, int top, int right, int bottom)const {
		return m_vSum[bottom + 1][right + 1] - m_vSum[top][right + 1] - m_vSum[bottom + 1][left] + m_vSum[top][left];
	}
	T GetTopLeft(int bottom, int right) { return m_vSum[bottom + 1][right + 1]; }
	T GetBottomRight(int top, int left) { return Get(left, top, m_iColCnt - 1, m_iRowCnt - 1); }
	vector<vector<T>> m_vSum;
	const int m_iRowCnt, m_iColCnt;
};

template<class INDEX_TYPE>
class CBinarySearch
{
public:
	CBinarySearch(INDEX_TYPE iMinIndex, INDEX_TYPE iMaxIndex, INDEX_TYPE tol = 1) :m_iMin(iMinIndex), m_iMax(iMaxIndex), m_iTol(tol) {}
	template<class _Pr>
	INDEX_TYPE FindFrist(_Pr pr)
	{
		auto left = m_iMin - m_iTol;
		auto rightInclue = m_iMax;
		while (rightInclue - left > m_iTol)
		{
			const auto mid = left + (rightInclue - left) / 2;
			if (pr(mid))
			{
				rightInclue = mid;
			}
			else
			{
				left = mid;
			}
		}
		return rightInclue;
	}
	template<class _Pr>
	INDEX_TYPE FindEnd(_Pr pr)
	{
		INDEX_TYPE leftInclude = m_iMin;
		INDEX_TYPE right = m_iMax + m_iTol;
		while (right - leftInclude > m_iTol)
		{
			const auto mid = leftInclude + (right - leftInclude) / 2;
			if (pr(mid))
			{
				leftInclude = mid;
			}
			else
			{
				right = mid;
			}
		}
		return leftInclude;
	}
protected:
	const INDEX_TYPE m_iMin, m_iMax, m_iTol;
};

class Solution {
public:
	int Ans(const int Need, vector<pair<int, int>>& cr) {
		const int N = cr.size();
		vector<int> cs, rs;
		for (const auto& [c, r] : cr) {
			cs.emplace_back(c);
			rs.emplace_back(r);
		}
		CDiscretize<int> cdis(cs), rdis(rs);
		const int R = rdis.size(), C = cdis.size();
		vector<vector<int>> pts(R, vector<int>(C));
		for (const auto& [c, r] : cr) {
			pts[rdis[r]][cdis[c]]++;
		}
		CPreSum2 preSum(R, C, [&](const int r, const int c) {return pts[r][c]; });
		auto Check = [&](int mid) {
			for (int r = 0, r1 = 0; r < R; r++) {
				while ((r1 + 1 < R) && (rdis.m_nums[r1 + 1] - rdis.m_nums[r] + 1 <= mid))
				{
					r1++;
				}
				for (int c = 0, c1 = 0; c < C; c++) {
					while ((c1 + 1 < C) && (cdis.m_nums[c1 + 1] - cdis.m_nums[c] + 1 <= mid)) {
						c1++;
					}
					if (preSum.Get(c, r, c1, r1) >= Need) { return true; }
				}
			}
			return false;
		};
		return CBinarySearch<int>(1, 10'000).FindFrist(Check);
	}
};

int main() {
#ifdef _DEBUG
	freopen("a.in", "r", stdin);
#endif // DEBUG	
	int C;
	cin >> C;
	auto xy = Read<pair<int,int>>();
	auto res = Solution().Ans(C,xy);
	cout << res;
#ifdef _DEBUG		
	//printf(",n=%d,", n);
	Out(xy, "xy=");	
#endif // DEBUG	
	return 0;
}

单元测试

	vector<pair<int, int>> xy;
		TEST_METHOD(TestMethod11)
		{
			xy = { {1,2},{2,1},{4,1},{5,2} };
			auto res = Solution().Ans(3,xy);
			AssertEx(4, res);
		}

扩展阅读

我想对大家说的话
工作中遇到的问题，可以按类别查阅鄙人的算法文章，请点击《算法与数据汇总》。
学习算法：按章节学习《喜缺全书算法册》，大量的题目和测试用例，打包下载。重视操作
有效学习：明确的目标 及时的反馈 拉伸区（难度合适） 专注
闻缺陷则喜(喜缺)是一个美好的愿望，早发现问题，早修改问题，给老板节约钱。
子墨子言之：事无终始，无务多业。也就是我们常说的专业的人做专业的事。
如果程序是一条龙，那算法就是他的是睛
失败+反思=成功 成功+反思=成功

视频课程

先学简单的课程，请移步CSDN学院，听白银讲师（也就是鄙人）的讲解。
https://edu.csdn.net/course/detail/38771
如何你想快速形成战斗了，为老板分忧，请学习C#入职培训、C++入职培训等课程
https://edu.csdn.net/lecturer/6176

测试环境

操作系统：win7 开发环境： VS2019 C++17
或者 操作系统：win10 开发环境： VS2022 C++17
如无特殊说明，本算法用**C++**实现。