ACM--單調棧--Bad Hair Day--POJ--3250--水
Description
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
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.
Input
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.
Output
Sample Input
6 10 3 7 4 12 2
Sample Output
5
================================華麗的分割線=============================
單調棧的入門題:單調棧就是維護一個棧,棧中的元素都保持著單調遞增或遞減的順序。
題目意思:有n只牛站在一排,給出隊伍中每隻牛的高度,每隻牛隻能看到它右邊比它矮的牛,求所有的牛能看到的牛數之和。
當我們新加入一個高度值時,如果棧中存在元素小於新加入的高度值,那這個牛肯定看不見這個高度的牛,就把這個元素彈棧。
每次加入新元素,並執行完彈出操作後,棧中元素個數便是可以看見這個牛的“牛數”。
#include<iostream>
#include<cstdio>
#include<stack>
using namespace std;
int main(void){
int n;
stack <long long>sta;
if(scanf("%d",&n)!=EOF){
long long result=0;
long long x;
cin>>x;
//將第一個元素入棧
sta.push(x);
for(int i=1;i<n;i++){
cin>>x;
while(!sta.empty()&&sta.top()<=x){
sta.pop();
}
result+=sta.size();
sta.push(x);
}
cout<<result<<endl;
//清空棧
while(!sta.empty()){
sta.pop();
}
}
return 0;
}