Codeforces 602B Approximating a Constant Range（想法題）

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B. Approximating a Constant Range

When Xellos was doing a practice course in university, he once had to measure the intensity of an effect that slowly approached equilibrium. A good way to determine the equilibrium intensity would be choosing a sufficiently large number of consecutive data points that seems as constant as possible and taking their average. Of course, with the usual sizes of data, it‘s nothing challenging — but why not make a similar programming contest problem while we‘re at it?

You‘re given a sequence of n data points a1, ...,an. There aren‘t any big jumps between consecutive data points — for each 1 ≤i<n, it‘s guaranteed that |ai+ 1-ai| ≤ 1.

A range [l,r] of data points is said to be almost constant if the difference between the largest and the smallest value in that range is at most 1. Formally, let M be the maximum and m the minimum value of ai for l≤i≤r; the range [l,r] is almost constant if M-m≤ 1.

Find the length of the longest almost constant range.

Input

The first line of the input contains a single integer n (2 ≤n≤ 100 000) — the number of data points.

The second line contains n integers a1,a2, ...,an (1 ≤ai≤ 100 000).

Output

Print a single number — the maximum length of an almost constant range of the given sequence.

Sample test(s)

input

5

1 2 3 3 2

output

4

input

11

5 4 5 5 6 7 8 8 8 7 6

output

5

 

來自 <http://codeforces.com/contest/602/problem/B>

Codeforces Round #333 (Div. 2)

 

 

【題意】：

n個數，相鄰數的差不超過1.

求最長的區間，使得極差不超過1.

 

【解題思路】：

對於X，包含X的合法區間需要考慮X-1 X+1 X+2 X-2的位置：

用數組P[i]記錄下至此 i 出現的最大位置；

若X-1的最大位置大於X+1的，則考慮X+1和X-2的位置即可；

相反，只需要考慮X-1和X+2的位置。

 

 1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<cmath> 5 #include<algorithm> 6 #define inf 0x3f3f3f3f 7 #define LL long long 8 #define maxn 110000 9 #define IN freopen("in.txt","r",stdin);10 using namespace std;11 12 int n;13 int p[maxn];14 15 int main(int argc, char const *argv[])16 {17     //IN;18 19     while(scanf("%d",&n)!=EOF)20     {21         int ans = -1;22         memset(p, 0, sizeof(p));23 24         for(int i=1;i<=n;i++){25             int x;scanf("%d",&x);26 27             if(p[x-1]>p[x+1]) ans = max(ans, i-max(p[x+1],p[x-2]));28             else ans = max(ans, i-max(p[x+2],p[x-1]));29 30             p[x] = i;31         }32 33         printf("%d\n", ans);34     }35 36     return 0;37 }

 

Codeforces 602B Approximating a Constant Range（想法題）