codeforcesRound#259(div2)B解题报告

B. Little Pony and Sort by Shift time limit per test 1 second memory limit per test 256 megabytes input standard input output standard output One day, Twilight Sparkle is interested in how to sort a sequence of integers a 1 ,? a 2 ,?...,?

B. Little Pony and Sort by Shift

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

One day, Twilight Sparkle is interested in how to sort a sequence of integers a 1 ,? a 2 ,?...,? a n in non-decreasing order. Being a young unicorn, the only operation she can perform is a unit shift. That is, she can move the last element of the sequence to its beginning:

a 1 ,? a 2 ,?...,? a n ?→? a n ,? a 1 ,? a 2 ,?...,? a n ?-?1 .

Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

Input

The first line contains an integer n (2?≤? n ?≤?10 5 ) . The second line contains n integer numbers a 1 ,? a 2 ,?...,? a n (1?≤? a i ?≤?10 5 ) .

Output

If it's impossible to sort the sequence output -1 . Otherwise output the minimum number of operations Twilight Sparkle needs to sort it.

Sample test(s)

input

2
2 1
 

output

1
 

input

3
1 3 2
 

output

-1
 

input

2
1 2
 

output

0 

题目大意：

给出N个数字，可以每一次将最后一个数字移动到最前面，要求最终状态是一个单调非递减的序列，求最少需要花多少次操作。如若无法达到目标则 输出“-1"。

解法：

也是一道很easy的编程基础题，找出两队单调非递减序列，分别为1~x 和 x+1~y，判断这两队是否覆盖整串数字，且a[n]

更简单的一种做法就是，将a[1]~a[n]复制一遍，拓展到a[1]~a[2*n]，然后在1 ~ 2*n里面找，是否有一串单调不递减的个数为n的序列。

代码：

#include  
#define N_max 123456

int n, x, y, cnt;
int a[N_max];

void init() {
	scanf("%d", &n);
	for (int i = 1; i   a[i+1]) {
			x = i;
			break;
		}

	if (x == n)
		y = n;
	else
		for (int i = x+1; i   a[i+1]) {
				y = i;
				break;
			}

	if (x == n)
		printf("0\n");
	else if (y == n && a[y]  

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