#[allow(unused_imports)] use std::cmp::*; #[allow(unused_imports)] use std::collections::*; // https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8 macro_rules! input { ($($r:tt)*) => { let stdin = std::io::stdin(); let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock())); let mut next = move || -> String{ bytes.by_ref().map(|r|r.unwrap() as char) .skip_while(|c|c.is_whitespace()) .take_while(|c|!c.is_whitespace()) .collect() }; input_inner!{next, $($r)*} }; } macro_rules! input_inner { ($next:expr) => {}; ($next:expr,) => {}; ($next:expr, $var:ident : $t:tt $($r:tt)*) => { let $var = read_value!($next, $t); input_inner!{$next $($r)*} }; } macro_rules! read_value { ($next:expr, [ $t:tt ; $len:expr ]) => { (0..$len).map(|_| read_value!($next, $t)).collect::>() }; ($next:expr, usize1) => (read_value!($next, usize) - 1); ($next:expr, $t:ty) => ($next().parse::<$t>().expect("Parse error")); } /** * Segment Tree. This data structure is useful for fast folding on intervals of an array * whose elements are elements of monoid I. Note that constructing this tree requires the identity * element of I and the operation of I. * Verified by: yukicoder No. 259 (http://yukicoder.me/submissions/100581) * AGC015-E (http://agc015.contest.atcoder.jp/submissions/1461001) * yukicoder No. 833 (https://yukicoder.me/submissions/703521) */ struct SegTree { n: usize, orign: usize, dat: Vec, op: BiOp, e: I, } impl SegTree where BiOp: Fn(I, I) -> I, I: Copy { pub fn new(n_: usize, op: BiOp, e: I) -> Self { let mut n = 1; while n < n_ { n *= 2; } // n is a power of 2 SegTree {n: n, orign: n_, dat: vec![e; 2 * n - 1], op: op, e: e} } /* ary[k] <- v */ pub fn update(&mut self, idx: usize, v: I) { debug_assert!(idx < self.orign); let mut k = idx + self.n - 1; self.dat[k] = v; while k > 0 { k = (k - 1) / 2; self.dat[k] = (self.op)(self.dat[2 * k + 1], self.dat[2 * k + 2]); } } /* [a, b) (note: half-inclusive) * http://proc-cpuinfo.fixstars.com/2017/07/optimize-segment-tree/ */ #[allow(unused)] pub fn query(&self, mut a: usize, mut b: usize) -> I { debug_assert!(a <= b); debug_assert!(b <= self.orign); let mut left = self.e; let mut right = self.e; a += self.n - 1; b += self.n - 1; while a < b { if (a & 1) == 0 { left = (self.op)(left, self.dat[a]); } if (b & 1) == 0 { right = (self.op)(self.dat[b - 1], right); } a = a / 2; b = (b - 1) / 2; } (self.op)(left, right) } } trait Change { fn chmax(&mut self, x: Self); fn chmin(&mut self, x: Self); } impl Change for T { fn chmax(&mut self, x: T) { if *self < x { *self = x; } } fn chmin(&mut self, x: T) { if *self > x { *self = x; } } } // #{(i, j) | j - i >= 3, kind[i] == kind[j] == lt, a[j] < b[i] < b[j] } // + #{(i, j) | j - i >= 3, kind[i] == kind[j] == gt, b[j] < b[i] <= a[j] } fn pat1(a: &[usize], b: &[(usize, bool)]) -> i64 { let n = a.len(); let mut st0 = SegTree::new(n, |x, y| x + y, 0); let mut st1 = SegTree::new(n, |x, y| x + y, 0); let mut ans = 0; for i in 3..n { { let (b, lt) = b[i - 3]; if lt { let val = st1.query(b, b + 1); st1.update(b, val + 1); } else { let val = st0.query(b, b + 1); st0.update(b, val + 1); } } let (b, lt) = b[i]; if lt { ans += st1.query(a[i] + 1, b); } else { ans += st0.query(b + 1, a[i]); } } ans } // #{(i, j) | |j - i| >= 3, (kind[i], kind[j]) == (lt, gt), a[j] < b[i], b[j] < a[i] } fn pat2(a: &[usize], b: &[(usize, bool)]) -> i64 { let n = a.len(); let mut add = vec![vec![]; n]; let mut q = vec![vec![]; n]; for i in 0..n { let a = a[i]; let (b, lt) = b[i]; if lt { add[b].push(a); } else { q[a].push(b); } } let mut st = SegTree::new(n, |x, y| x + y, 0); let mut ans = 0; for i in (0..n).rev() { for &idx in &q[i] { ans += st.query(idx + 1, n); } for &idx in &add[i] { let val = st.query(idx, idx + 1); st.update(idx, val + 1); } } for i in 0..n - 1 { let (ai, bi, aj, bj) = if b[i].1 { (a[i], b[i].0, a[i + 1], b[i + 1].0) } else { (a[i + 1], b[i + 1].0, a[i], b[i].0) }; if aj < bi && bj < ai { ans -= 1; } } ans } // #{(i, j) | j - i <= 2, swappable } fn pat3(a: &[usize]) -> i64 { let n = a.len(); let mut a = a.to_vec(); let mut ans = 0; for i in 0..n - 2 { for j in i + 1..min(i + 3, n) { a.swap(i, j); let mut ok = true; for k in max(1, max(i, 1) - 1)..min(j + 1, n - 2) + 1 { ok &= (a[k] > a[k + 1]) == (a[k] > a[k - 1]); } if ok { ans += 1; } a.swap(i, j); } } ans } fn main() { input! { n: usize, a: [usize1; n], } let mut b = vec![(0, false); n]; for i in 0..n { let lt = if i > 0 { a[i - 1] > a[i] } else { a[i + 1] > a[i] }; let mut x = if i > 0 { a[i - 1] } else { a[i + 1] }; if i + 1 < n { if lt { x = min(x, a[i + 1]); } else { x = max(x, a[i + 1]); } } b[i] = (x, lt); } // eprintln!("{:?}", b); let mut ans = pat1(&a, &b); ans += pat2(&a, &b); ans += pat3(&a); let mut a = a; a.reverse(); b.reverse(); ans += pat1(&a, &b); println!("{}", ans); }