


codeforces Round #259(div2) BProblem Solution Report_html/css_WEB-ITnose
Jun 24, 2016 am 11:55 AM
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?a1,?a2,?...,?an?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:
a1,?a2,?...,?an?→?an,?a1,?a2,?...,?an?-?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?≤?105). The second line contains?n?integer numbers?a1,?a2,?...,?an?(1?≤?ai?≤?105).
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
22 1
output
input
31 3 2
output
-1
input
21 2
output
題目大意:
給出N個(gè)數(shù)字,可以每一次將最后一個(gè)數(shù)字移動(dòng)到最前面,要求最終狀態(tài)是一個(gè)單調(diào)非遞減的序列,求最少需要花多少次操作。如若無法達(dá)到目標(biāo)則輸出“-1"。
解法:
也是一道很easy的編程基礎(chǔ)題,找出兩隊(duì)單調(diào)非遞減序列,分別為1~x 和 x 1~y,判斷這兩隊(duì)是否覆蓋整串?dāng)?shù)字,且a[n] <= a[1]。
更簡單的一種做法就是,將a[1]~a[n]復(fù)制一遍,拓展到a[1]~a[2*n],然后在1 ~ 2*n里面找,是否有一串單調(diào)不遞減的個(gè)數(shù)為n的序列。
代碼:
#include#define N_max 123456int n, x, y, cnt;int a[N_max];void init() { scanf("%d", &n); for (int i = 1; i <= n; i++) scanf("%d", &a[i]);}void solve() { for (int i = 1; i <= n; i++) if (a[i] > a[i+1]) { x = i; break; } if (x == n) y = n; else for (int i = x+1; i <= n; i++) if (a[i] > a[i+1]) { y = i; break; } if (x == n) printf("0\n"); else if (y == n && a[y] <= a[1]) printf("%d\n", y-x); else printf("-1\n");}int main() { init(); solve();}

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