hash浅谈

刚刚做了poj3349和poj3274， 都是很水的题， 明天还要再做一些hash的题， 因为我这方面总是有问题。 对了我最大的问题还是搜索！！！巨简单的搜索题我就是写不对， 简直是不能要了！！！明天做poj2531,poj1416,poj2676,1129这几道搜索题吧， 必须多练习这些基础的东西， 其实真正比赛的时候那些高深的算法基本上是没有什么用的， 学那些也只是为了提高思维的深度和广度而已。

言归正传， 基础的hash还是非常的简单的，主要就是找一个适合mod就行了， 还有就是数组最好是动态建立的， 就要用vector了， 我以前没有用过这个的。主要内容就是：

头文件：#include <vector> 

申请：vector<int>hash[mod];

循环时的变量： for(vector<int>::size_type j = 0; j < hash[i].size(); j ++)

加入: hash[i].push_back(j);

 

 

 

cplusplus中的

Modifiers:

assign

Assign vector content (public member function )

push_back

Add element at the end (public member function )

pop_back

Delete last element (public member function )

insert

Insert elements (public member function )

erase

Erase elements (public member function )

swap

Swap content (public member function )

clear

Clear content (public member function )

emplace

Construct and insert element (public member function )

emplace_back

Construct and insert element at the end (public member function )

 

然后贴下这两个代码

poj3349

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <vector>
#define MAXX 10000005
#define MAXN 100005
#define mod 900001
using namespace std;
int n, snow[MAXX][6];
vector<int>hash[mod + 5]; 
bool Same(int a, int b){
	for(int i = 0; i < 6; i ++){
		if((snow[a][1] == snow[b][(i + 1)%6] && snow[a][2] == snow[b][(i + 2)%6] && snow[a][3] == snow[b][(i + 3)%6] && snow[a][4] == snow[b][(i + 4)%6] && snow[a][5] == snow[b][(i + 5)%6] && snow[a][0] == snow[b][(i + 6)%6]) || 
			(snow[a][1] == snow[b][(i + 1) % 6] && snow[a][2] == snow[b][i] && snow[a][3] == snow[b][(i + 5) % 6] && snow[a][4] == snow[b][(i + 4) % 6] && snow[a][5] == snow[b][(i + 3)% 6] && snow[a][0] == snow[b][(i + 2) % 6])
		)	return 1;
	}	return 0;
}
int main()
{
	scanf("%d", &n);
	for(int i = 1; i <= n; i ++){
		int ll = 0;
		for(int j = 0; j < 6; j ++){
			scanf("%d", &snow[i][j]);
			ll += snow[i][j];	
			ll %= mod;
		}
		for(vector<int>::size_type j = 0; j < hash[ll].size(); j ++){
		//	printf("%d      %d\n", hash[ll][j], i);
			if(Same(hash[ll][j], i)){
				puts("Twin snowflakes found."); return 0;	
			}	
		}	hash[ll].push_back(i);
	}
	puts("No two snowflakes are alike.");
    //system("pause");
    return 0;
}

poj3274

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <vector>
#define mod 99911
using namespace std;
int n, k, f[100005][32], sum[100005][32], c[100005][32], maxx;
vector<int>hash[mod + 5]; 
bool cmp(int a, int b){
	for(int j = 1; j <= k; j ++)
		if(c[a][j] != c[b][j])return 0;
	return 1;	
}
void hashh(int ci){
	int key = 0;
	for(int i = 2; i <= k; i ++)
		key += c[ci][i];
	key = abs(key) % mod;
	for(vector<int>::size_type j = 0; j < hash[key].size(); j ++){
		if(cmp(ci, hash[key][j])){
			int dis = ci - hash[key][j];
			maxx = max(maxx, dis);	
			return ;
		}
	 }
	 hash[key].push_back(ci);	
}
int main()
{
	scanf("%d%d", &n, &k);
	int a;
	for(int i = 1; i <= n; i ++){
		scanf("%d", &a);
		for(int j = 1; j <= k; j ++){
			f[i][j] = a % 2; a /= 2;
			sum[i][j] = sum[i - 1][j] + f[i][j];
			if(j == 1)continue;
			c[i][j] = sum[i][j] - sum[i][1];
		}	
	}
	for(int i = 0; i <= n; i ++)hashh(i);
	printf("%d\n", maxx);
    //system("pause");
    return 0;
}