尺取 Poj3601

http://poj.org/problem?id=3061

Subsequence

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 20744		Accepted: 8849

Description

A sequence of N positive integers (10 < N < 100 000), each of them less than or equal 10000, and a positive integer S (S < 100 000 000) are given. Write a program to find the minimal length of the subsequence of consecutive elements of the sequence, the sum of which is greater than or equal to S.

Input

The first line is the number of test cases. For each test case the program has to read the numbers N and S, separated by an interval, from the first line. The numbers of the sequence are given in the second line of the test case, separated by intervals. The input will finish with the end of file.

Output

For each the case the program has to print the result on separate line of the output file.if no answer, print 0.

Sample Input

2
10 15
5 1 3 5 10 7 4 9 2 8
5 11
1 2 3 4 5

Sample Output

2
3

题意：给定长度为n的数列整数a0,a1,a2,a3 ..... an-1以及整数S。求出综合不小于S的连续子序列的长度的最小值。如果解不存在，则输出0。

思路：其实相当于当不满足条件就入队，然后得到队列长度，再将队首元素出队，再进行下一次的入队，直到满足条件再次出队，并且将这一次的长度与历史最短长度进行取舍，最后扫到最后的元素却无法再满足入队条件的时候就结束，此时用O(n)的时间就可以得到答案。

我的ac代码（较混乱）：

#include <map>
#include <set>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>

using namespace std;
typedef long long ll;
const ll INF=0xfffffff;
ll N,n,m;
ll a[100000+10];
			
int main() 
{
	std::ios::sync_with_stdio(false);
	cin>>N;
	while(N--){
		ll sum=0,f=0;
		ll ans=INF;
		cin>>n>>m;
		for(int i=0;i<n;i++) cin>>a[i];
		for(int i=0;i<n;i++){
			sum+=a[i];
			f++;
			while(sum>=m&&sum){
				ans=min(ans,f);
				f--;
				sum-=a[i-f];
			}
		}
		if(ans==INF) puts("0");
		else if(m==0) puts("1");
		else cout<<ans<<endl;
	}
}

模板（用了首尾值）：

#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<iostream>
#include<algorithm>
 
typedef long long ll;
using namespace std;
const int INF=0x3f3f3f3f;
int a[100010];
 
int main(){
	ios::sync_with_stdio(false);
	int N;
	cin>>N;	
	while(N--){
		int n,m;
		int ans=INF;
		cin>>n>>m;
		for(int i=1;i<=n;i++) cin>>a[i];
		int i=1,j=1;//i为序列的首，j为序列的尾 
		int sum = 0;		
		while(1)
		{
			while(j<=n&&sum<m)
			{ //找到第一个末位置使序列之和大于s 
				sum+=a[j];
				j++;
			} 
			if(sum<m)	break;    
			//从i一直到给出数列的尾部的所有和都不能大于等于s的情况，之后的所有和只可能比当前小，退出 	
			ans=min(ans,j-i);
			sum-=a[i];
			i++; 
		} 		
		if(ans==INF) ans=0;	
		cout<<ans<<endl;
	}
	return 0;
}