BZOJ1660: [Usaco2006 Nov]Bad Hair Day 乱发节(单调栈)

#include<bits/stdc++.h> 
//#define int long long 
#define LL long long 
using namespace std;
const int MAXN = 1e6 + 10;
inline int read() {
    char c = getchar(); int 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;
}
int N, a[MAXN], st[MAXN], top;
signed main() {
    N = read();
    for(int i = 1; i <= N; i++) a[i] = read();
    reverse(a + 1, a + N + 1);
    LL ans = 0;
    for(int i = 1; i <= N; i++) {
        while(top && a[i] > a[st[top]]) top--;
        ans += i - st[top] - 1;
        st[++top] = i;
    }
    cout << ans;
    return 0;
}
/*
5
5 5 5 5 5
*/