洛谷P2866 [USACO06NOV]糟糕的一天Bad Hair Day(单调栈)
题目描述
Some of Farmer John's N cows (1 ≤ N ≤ 80,000) are having a bad hair day! Since each cow is self-conscious about her messy hairstyle, FJ wants to count the number of other cows that can see the top of other cows' heads.
Each cow i has a specified height hi (1 ≤ hi ≤ 1,000,000,000) and is standing in a line of cows all facing east (to the right in our diagrams). Therefore, cow i can see the tops of the heads of cows in front of her (namely cows i+1, i+2, and so on), for as long as these cows are strictly shorter than cow i.
Consider this example:
=
= =
= - = Cows facing right -->
= = =
= - = = =
= = = = = =
1 2 3 4 5 6 Cow#1 can see the hairstyle of cows #2, 3, 4
Cow#2 can see no cow's hairstyle
Cow#3 can see the hairstyle of cow #4
Cow#4 can see no cow's hairstyle
Cow#5 can see the hairstyle of cow 6
Cow#6 can see no cows at all!
Let ci denote the number of cows whose hairstyle is visible from cow i; please compute the sum of c1 through cN.For this example, the desired is answer 3 + 0 + 1 + 0 + 1 + 0 = 5.
农民约翰的某N(1 < N < 80000)头奶牛正在过乱头发节!由于每头牛都意识到自己凌乱不堪 的发型,约翰希望统计出能够看到其他牛的头发的牛的数量.
每一头牛i有一个高度所有N头牛面向东方排成一排,牛N在最前面,而 牛1在最后面.第i头牛可以看到她前面的那些牛的头,只要那些牛的高度严格小于她的高度,而且 中间没有比hi高或相等的奶牛阻隔.
让N表示第i头牛可以看到发型的牛的数量;请输出Ci的总和
输入输出格式
输入格式:
Line 1: The number of cows, N.
Lines 2..N+1: Line i+1 contains a single integer that is the height of cow i.
输出格式:
Line 1: A single integer that is the sum of c1 through cN.
输入输出样例
6 10 3 7 4 12 2
5
只要能看出是单调栈
这题就做完了
#include<cstdio> #define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++) #define LL long long char buf[1<<21],*p1=buf,*p2=buf; const int MAXN = 80001; inline LL read() { char c = getchar(); LL x = 0, f = 1; while(c < '0' || c > '9') {if(c == '-') f = -1;c = getchar();} while(c >= '0' && c <= '9') {x = x * 10 + c - '0';c = getchar();} return x * f; } LL a[MAXN]; int Ans[MAXN], s[MAXN]; int N, top = 0; int main() { N = read(); for(int i = 1; i <= N; i++) a[i] = read(); s[0] = N + 1; LL ans = 0; for(int i = N; i >= 1; i--) { while(top > 0 && a[i] > a[ s[top] ]) top--; ans += (s[top] - i - 1); s[++top] = i; } printf("%lld", ans); return 0; }
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