A-Unimodal Array

题干

Array of integers is unimodal, if:
·it is strictly increasing in the beginning;
·after that it is constant;
·after that it is strictly decreasing.
The first block (increasing) and the last block (decreasing) may be absent. It is allowed that both of this blocks are absent.
For example, the following three arrays are unimodal: [5, 7, 11, 11, 2, 1], [4, 4, 2], [7], but the following three are not unimodal: [5, 5, 6, 6, 1], [1, 2, 1, 2], [4, 5, 5, 6].
Write a program that checks if an array is unimodal.

Input
The first line contains integer n (1 ≤ n ≤ 100) — the number of elements in the array.
The second line contains n integers a 1, a 2, …, a n (1 ≤ a i ≤ 1 000) — the elements of the array.

Output
Print “YES” if the given array is unimodal. Otherwise, print “NO”.
You can output each letter in any case (upper or lower).

题解

题目中要求的是一个左边单调增，中间恒定，右边单调减的数列，其中三种情况都可以缺失。
因此可以给每种情况附一个值。
单调增为 1，
恒定为 2，
单调减为 3，
由题意得，右边的值不能比左边的值低。
特殊情况：n=1时一定成立。

参考代码

#include
using namespace std;
int n,a[101],f[101];
int main()
{
	cin>>n;
	if(n==1)
	{
		cout<<"YES";
		return 0;
	}
	for(int i=1;i<=n;i++)	cin>>a[i];
	for(int i=2;i<=n;i++)
	{
		if(a[i-1]<a[i])	f[i]=1;
		if(a[i-1]==a[i])f[i]=2;
		if(a[i-1]>a[i])	f[i]=3;
		if(f[i]<f[i-1])
		{
			cout<<"NO";
			return 0;
		}
	}
	cout<<"YES";
}