Problem:
Score : 600 points

Problem Statement

A subsequence of a string S is a string that can be obtained by deleting zero or more characters from S without changing the order of the remaining characters.
For example, arc, artistic and (an empty string) are all subsequences of artistic; abc and ci are not.

You are given a string A consisting of lowercase English letters.
Find the shortest string among the strings consisting of lowercase English letters that are not subsequences of A.
If there are more than one such string, find the lexicographically smallest one among them.

Constraints

1 \leq |A| \leq 2 \times 10^5

A consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

A

Output

Print the lexicographically smallest string among the shortest strings consisting of lowercase English letters that are not subsequences of A.

Sample Input 1

atcoderregularcontest

Sample Output 1

b

The string atcoderregularcontest contains a as a subsequence, but not b.

Sample Input 2

abcdefghijklmnopqrstuvwxyz

Sample Output 2

aa

Sample Input 3

frqnvhydscshfcgdemurlfrutcpzhopfotpifgepnqjxupnskapziurswqazdwnwbgdhyktfyhqqxpoidfhjdakoxraiedxskywuepzfniuyskxiyjpjlxuqnfgmnjcvtlpnclfkpervxmdbvrbrdn

Sample Output 3

aca

Solution in Rust (1.42.0) (judge verdict: Accepted)

pub trait Zero: PartialEq + Sized {
    fn zero() -> Self;
    #[inline]
    fn is_zero(&self) -> bool {
        self == &Self::zero()
    }
}
pub trait One: PartialEq + Sized {
    fn one() -> Self;
    #[inline]
    fn is_one(&self) -> bool {
        self == &Self::one()
    }
}
macro_rules !zero_one_impls {($({$Trait :ident $method :ident $($t :ty ) *,$e :expr } ) *) =>{$($(impl $Trait for $t {#[inline ] fn $method () ->Self {$e } } ) *) *} ;}
zero_one_impls !({Zero zero u8 u16 u32 u64 usize i8 i16 i32 i64 isize u128 i128 ,0 } {Zero zero f32 f64 ,0. } {One one u8 u16 u32 u64 usize i8 i16 i32 i64 isize u128 i128 ,1 } {One one f32 f64 ,1. } ) ;
pub trait IterScan: Sized {
    type Output;
    fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output>;
}
pub trait MarkedIterScan: Sized {
    type Output;
    fn mscan<'a, I: Iterator<Item = &'a str>>(self, iter: &mut I) -> Option<Self::Output>;
}
#[derive(Clone, Debug)]
pub struct Scanner<'a> {
    iter: std::str::SplitAsciiWhitespace<'a>,
}
mod scanner_impls {
    use super::*;
    impl<'a> Scanner<'a> {
        #[inline]
        pub fn new(s: &'a str) -> Self {
            let iter = s.split_ascii_whitespace();
            Self { iter }
        }
        #[inline]
        pub fn scan<T: IterScan>(&mut self) -> <T as IterScan>::Output {
            <T as IterScan>::scan(&mut self.iter).expect("scan error")
        }
        #[inline]
        pub fn mscan<T: MarkedIterScan>(&mut self, marker: T) -> <T as MarkedIterScan>::Output {
            marker.mscan(&mut self.iter).expect("scan error")
        }
        #[inline]
        pub fn scan_vec<T: IterScan>(&mut self, size: usize) -> Vec<<T as IterScan>::Output> {
            (0..size)
                .map(|_| <T as IterScan>::scan(&mut self.iter).expect("scan error"))
                .collect()
        }
        #[inline]
        pub fn iter<'b, T: IterScan>(&'b mut self) -> ScannerIter<'a, 'b, T> {
            ScannerIter {
                inner: self,
                _marker: std::marker::PhantomData,
            }
        }
    }
    macro_rules !iter_scan_impls {($($t :ty ) *) =>{$(impl IterScan for $t {type Output =Self ;#[inline ] fn scan <'a ,I :Iterator <Item =&'a str >>(iter :&mut I ) ->Option <Self >{iter .next () ?.parse ::<$t >() .ok () } } ) *} ;}
    iter_scan_impls !(char u8 u16 u32 u64 usize i8 i16 i32 i64 isize f32 f64 u128 i128 String ) ;
    macro_rules !iter_scan_tuple_impl {($($T :ident ) *) =>{impl <$($T :IterScan ) ,*>IterScan for ($($T ,) *) {type Output =($(<$T as IterScan >::Output ,) *) ;#[inline ] fn scan <'a ,It :Iterator <Item =&'a str >>(_iter :&mut It ) ->Option <Self ::Output >{Some (($(<$T as IterScan >::scan (_iter ) ?,) *) ) } } } ;}
    iter_scan_tuple_impl!();
    iter_scan_tuple_impl!(A);
    iter_scan_tuple_impl !(A B ) ;
    iter_scan_tuple_impl !(A B C ) ;
    iter_scan_tuple_impl !(A B C D ) ;
    iter_scan_tuple_impl !(A B C D E ) ;
    iter_scan_tuple_impl !(A B C D E F ) ;
    iter_scan_tuple_impl !(A B C D E F G ) ;
    iter_scan_tuple_impl !(A B C D E F G H ) ;
    iter_scan_tuple_impl !(A B C D E F G H I ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J K ) ;
    pub struct ScannerIter<'a, 'b, T> {
        inner: &'b mut Scanner<'a>,
        _marker: std::marker::PhantomData<fn() -> T>,
    }
    impl<'a, 'b, T: IterScan> Iterator for ScannerIter<'a, 'b, T> {
        type Item = <T as IterScan>::Output;
        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            <T as IterScan>::scan(&mut self.inner.iter)
        }
    }
}
pub mod marker {
    use super::*;
    use std::{iter::FromIterator, marker::PhantomData};
    #[derive(Debug, Copy, Clone)]
    pub struct Usize1;
    impl IterScan for Usize1 {
        type Output = usize;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            Some(<usize as IterScan>::scan(iter)?.checked_sub(1)?)
        }
    }
    #[derive(Debug, Copy, Clone)]
    pub struct Chars;
    impl IterScan for Chars {
        type Output = Vec<char>;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            Some(iter.next()?.chars().collect())
        }
    }
    #[derive(Debug, Copy, Clone)]
    pub struct CharsWithBase(pub char);
    impl MarkedIterScan for CharsWithBase {
        type Output = Vec<usize>;
        #[inline]
        fn mscan<'a, I: Iterator<Item = &'a str>>(self, iter: &mut I) -> Option<Self::Output> {
            Some(
                iter.next()?
                    .chars()
                    .map(|c| (c as u8 - self.0 as u8) as usize)
                    .collect(),
            )
        }
    }
    #[derive(Debug, Copy, Clone)]
    pub struct Collect<T: IterScan, B: FromIterator<<T as IterScan>::Output>> {
        size: usize,
        _marker: PhantomData<fn() -> (T, B)>,
    }
    impl<T: IterScan, B: FromIterator<<T as IterScan>::Output>> Collect<T, B> {
        pub fn new(size: usize) -> Self {
            Self {
                size,
                _marker: PhantomData,
            }
        }
    }
    impl<T: IterScan, B: FromIterator<<T as IterScan>::Output>> MarkedIterScan for Collect<T, B> {
        type Output = B;
        #[inline]
        fn mscan<'a, I: Iterator<Item = &'a str>>(self, iter: &mut I) -> Option<Self::Output> {
            Some(
                (0..self.size)
                    .map(|_| <T as IterScan>::scan(iter).expect("scan error"))
                    .collect::<B>(),
            )
        }
    }
}
#[macro_export]
macro_rules !min {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::min ($e ,min !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmin {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::min ($dst ,min !($($src ) ,+) ) ;$dst =x ;} } ;}
#[macro_export]
macro_rules !max {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::max ($e ,max !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmax {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::max ($dst ,max !($($src ) ,+) ) ;$dst =x ;} } ;}
/// binary operaion: $T \circ T \to T$
pub trait Magma {
    /// type of operands: $T$
    type T: Clone + PartialEq;
    /// binary operaion: $\circ$
    fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T;
    #[inline]
    fn reverse_operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
        self.operate(y, x)
    }
}
/// $\forall a,\forall b,\forall c \in T, (a \circ b) \circ c = a \circ (b \circ c)$
pub trait Associative {}
/// associative binary operation
pub trait SemiGroup: Magma + Associative {}
impl<S: Magma + Associative> SemiGroup for S {}
/// $\exists e \in T, \forall a \in T, e \circ a = a \circ e = e$
pub trait Unital: Magma {
    /// identity element: $e$
    fn unit(&self) -> Self::T;
}
/// associative binary operation and an identity element
pub trait Monoid: SemiGroup + Unital {
    /// binary exponentiation: $x^n = x\circ\ddots\circ x$
    fn pow(&self, x: Self::T, n: usize) -> Self::T {
        let mut n = n;
        let mut res = self.unit();
        let mut base = x;
        while n > 0 {
            if n & 1 == 1 {
                res = self.operate(&res, &base);
            }
            base = self.operate(&base, &base);
            n = n >> 1;
        }
        res
    }
}
impl<M: SemiGroup + Unital> Monoid for M {}
/// $\exists e \in T, \forall a \in T, \exists b,c \in T, b \circ a = a \circ c = e$
pub trait Invertible: Magma {
    /// $a$ where $a \circ x = e$
    fn inverse(&self, x: &Self::T) -> Self::T;
    #[inline]
    fn rinv_operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
        self.operate(x, &self.inverse(y))
    }
}
/// associative binary operation and an identity element and inverse elements
pub trait Group: Monoid + Invertible {}
impl<G: Monoid + Invertible> Group for G {}
/// $\forall a,\forall b \in T, a \circ b = b \circ a$
pub trait Commutative {}
/// commutative monoid
pub trait AbelianMonoid: Monoid + Commutative {}
impl<M: Monoid + Commutative> AbelianMonoid for M {}
/// commutative group
pub trait AbelianGroup: Group + Commutative {}
impl<G: Group + Commutative> AbelianGroup for G {}
/// $\forall a \in T, a \circ a = a$
pub trait Idempotent {}
/// idempotent monoid
pub trait IdempotentMonoid: Monoid + Idempotent {}
impl<M: Monoid + Idempotent> IdempotentMonoid for M {}
#[derive(Clone, Debug)]
pub struct SegmentTree<M: Monoid> {
    n: usize,
    seg: Vec<M::T>,
    m: M,
}
impl<M: Monoid> SegmentTree<M> {
    pub fn new(n: usize, m: M) -> Self {
        let n = 1 << format!("{:b}", n - 1).len();
        let seg = vec![m.unit(); 2 * n];
        Self { n, seg, m }
    }
    pub fn from_vec(v: Vec<M::T>, m: M) -> Self {
        let n = 1 << format!("{:b}", v.len() - 1).len();
        let mut seg = vec![m.unit(); 2 * n];
        for (i, x) in v.into_iter().enumerate() {
            seg[n + i] = x;
        }
        for i in (1..n).rev() {
            seg[i] = m.operate(&seg[2 * i], &seg[2 * i + 1]);
        }
        Self { n, seg, m }
    }
    pub fn set(&mut self, k: usize, x: M::T) {
        debug_assert!(k < self.n);
        let mut k = k + self.n;
        self.seg[k] = x;
        k /= 2;
        while k > 0 {
            self.seg[k] = self.m.operate(&self.seg[2 * k], &self.seg[2 * k + 1]);
            k /= 2;
        }
    }
    pub fn update(&mut self, k: usize, x: M::T) {
        debug_assert!(k < self.n);
        let mut k = k + self.n;
        self.seg[k] = self.m.operate(&self.seg[k], &x);
        k /= 2;
        while k > 0 {
            self.seg[k] = self.m.operate(&self.seg[2 * k], &self.seg[2 * k + 1]);
            k /= 2;
        }
    }
    pub fn get(&self, k: usize) -> M::T {
        debug_assert!(k < self.n);
        self.seg[k + self.n].clone()
    }
    pub fn fold(&self, l: usize, r: usize) -> M::T {
        debug_assert!(l < self.n);
        debug_assert!(r <= self.n);
        let mut l = l + self.n;
        let mut r = r + self.n;
        let mut vl = self.m.unit();
        let mut vr = self.m.unit();
        while l < r {
            if l & 1 != 0 {
                vl = self.m.operate(&vl, &self.seg[l]);
                l += 1;
            }
            if r & 1 != 0 {
                r -= 1;
                vr = self.m.operate(&self.seg[r], &vr);
            }
            l /= 2;
            r /= 2;
        }
        self.m.operate(&vl, &vr)
    }
    pub fn fold_all(&self) -> M::T {
        self.seg[1].clone()
    }
    /// left most index [0, r) that satisfies monotonic condition
    pub fn lower_bound_all<F: Fn(&M::T) -> bool>(&self, f: F, r: usize) -> usize {
        if !f(&self.seg[1]) {
            return r;
        }
        let mut acc = self.m.unit();
        let mut pos = 1;
        while pos < self.n {
            pos *= 2;
            let y = self.m.operate(&acc, &self.seg[pos]);
            if !f(&y) {
                acc = y;
                pos += 1;
            }
        }
        std::cmp::min(pos - self.n, r)
    }
    /// left most index [l, r) that satisfies monotonic condition
    pub fn lower_bound<F: Fn(&M::T) -> bool>(&self, f: F, l: usize, r: usize) -> usize {
        let mut acc = self.m.unit();
        let mut pos = l + self.n;
        let mut lim = r + self.n;
        loop {
            let y = self.m.operate(&acc, &self.seg[pos]);
            if f(&y) {
                while pos < self.n {
                    pos *= 2;
                    let y = self.m.operate(&acc, &self.seg[pos]);
                    if !f(&y) {
                        acc = y;
                        pos += 1;
                    }
                }
                return std::cmp::min(pos - self.n, r);
            }
            let is_right = pos & 1 == 1;
            if pos == lim {
                return r;
            }
            pos /= 2;
            lim /= 2;
            if is_right {
                acc = y;
                pos += 1;
            }
        }
    }
    pub fn as_slice(&self) -> &[M::T] {
        &self.seg[self.n..]
    }
}
/// binary operation to select smaller element
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct MinOperation<T: Clone + Ord + MaximumBounded> {
    _marker: std::marker::PhantomData<fn() -> T>,
}
pub trait MaximumBounded {
    fn maximum() -> Self;
}
mod min_operation_impl {
    use super::*;
    macro_rules! impl_maximum_with_max {
        ($t :ty ,$max :expr ) => {
            impl MaximumBounded for $t {
                #[inline]
                fn maximum() -> Self {
                    $max
                }
            }
        };
    }
    impl_maximum_with_max!(usize, std::usize::MAX);
    impl_maximum_with_max!(u8, std::u8::MAX);
    impl_maximum_with_max!(u16, std::u16::MAX);
    impl_maximum_with_max!(u32, std::u32::MAX);
    impl_maximum_with_max!(u64, std::u64::MAX);
    impl_maximum_with_max!(isize, std::isize::MAX);
    impl_maximum_with_max!(i8, std::i8::MAX);
    impl_maximum_with_max!(i16, std::i16::MAX);
    impl_maximum_with_max!(i32, std::i32::MAX);
    impl_maximum_with_max!(i64, std::i64::MAX);
    macro_rules !impl_maximum_tuple {($($T :ident ) *) =>{impl <$($T :MaximumBounded ) ,*>MaximumBounded for ($($T ,) *) {#[inline ] fn maximum () ->Self {($(<$T as MaximumBounded >::maximum () ,) *) } } } ;}
    impl_maximum_tuple!();
    impl_maximum_tuple!(A);
    impl_maximum_tuple !(A B ) ;
    impl_maximum_tuple !(A B C ) ;
    impl_maximum_tuple !(A B C D ) ;
    impl_maximum_tuple !(A B C D E ) ;
    impl_maximum_tuple !(A B C D E F ) ;
    impl_maximum_tuple !(A B C D E F G ) ;
    impl_maximum_tuple !(A B C D E F G H ) ;
    impl_maximum_tuple !(A B C D E F G H I ) ;
    impl_maximum_tuple !(A B C D E F G H I J ) ;
    impl_maximum_tuple !(A B C D E F G H I J K ) ;
    impl<T: Clone + Ord + MaximumBounded> MinOperation<T> {
        pub fn new() -> Self {
            Self {
                _marker: std::marker::PhantomData,
            }
        }
    }
    impl<T: Clone + Ord + MaximumBounded> Magma for MinOperation<T> {
        type T = T;
        #[inline]
        fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
            x.min(y).clone()
        }
    }
    impl<T: Clone + Ord + MaximumBounded> Unital for MinOperation<T> {
        #[inline]
        fn unit(&self) -> Self::T {
            MaximumBounded::maximum()
        }
    }
    impl<T: Clone + Ord + MaximumBounded> Associative for MinOperation<T> {}
    impl<T: Clone + Ord + MaximumBounded> Commutative for MinOperation<T> {}
    impl<T: Clone + Ord + MaximumBounded> Idempotent for MinOperation<T> {}
}
/// |
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct BitOrOperation<T: Copy + PartialEq + BitOrIdentity> {
    _marker: std::marker::PhantomData<fn() -> T>,
}
pub trait BitOrIdentity: Sized + std::ops::BitOr<Output = Self> {
    fn all_zero() -> Self;
}
mod bitor_operation_impl {
    use super::*;
    #[macro_export(local_inner_macros)]
    macro_rules !impl_bitor_identity {([$($wh :tt ) *] ,$t :ty ,$all_zero :expr ) =>{impl <$($wh ) *>BitOrIdentity for $t {#[inline ] fn all_zero () ->Self {$all_zero } } } ;($t :ty ,$all_zero :expr ) =>{impl BitOrIdentity for $t {#[inline ] fn all_zero () ->Self {$all_zero } } } ;}
    impl_bitor_identity!(bool, false);
    impl_bitor_identity!(usize, 0usize);
    impl_bitor_identity!(u8, 0u8);
    impl_bitor_identity!(u16, 0u16);
    impl_bitor_identity!(u32, 0u32);
    impl_bitor_identity!(u64, 0u64);
    impl_bitor_identity!(isize, 0isize);
    impl_bitor_identity!(i8, 0i8);
    impl_bitor_identity!(i16, 0i16);
    impl_bitor_identity!(i32, 0i32);
    impl_bitor_identity!(i64, 0i64);
    impl<T: Copy + PartialEq + BitOrIdentity> BitOrOperation<T> {
        pub fn new() -> Self {
            Self {
                _marker: std::marker::PhantomData,
            }
        }
    }
    impl<T: Copy + PartialEq + BitOrIdentity> Magma for BitOrOperation<T> {
        type T = T;
        #[inline]
        fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
            *x | *y
        }
    }
    impl<T: Copy + PartialEq + BitOrIdentity> Unital for BitOrOperation<T> {
        #[inline]
        fn unit(&self) -> Self::T {
            BitOrIdentity::all_zero()
        }
    }
    impl<T: Copy + PartialEq + BitOrIdentity> Associative for BitOrOperation<T> {}
    impl<T: Copy + PartialEq + BitOrIdentity> Commutative for BitOrOperation<T> {}
    impl<T: Copy + PartialEq + BitOrIdentity> Idempotent for BitOrOperation<T> {}
}
fn main() {
    #![allow(unused_imports, unused_macros)]
    use std::io::{stdin, stdout, BufWriter, Read as _, Write as _};
    let mut _in_buf = Vec::new();
    stdin().read_to_end(&mut _in_buf).expect("io error");
    let _in_buf = unsafe { String::from_utf8_unchecked(_in_buf) };
    let mut scanner = Scanner::new(&_in_buf);
    macro_rules !scan {() =>{scan !(usize ) } ;(($($t :tt ) ,*) ) =>{($(scan !($t ) ) ,*) } ;([$t :ty ;$len :expr ] ) =>{scanner .scan_vec ::<$t >($len ) } ;([$t :tt ;$len :expr ] ) =>{(0 ..$len ) .map (|_ |scan !($t ) ) .collect ::<Vec <_ >>() } ;([$t :ty ] ) =>{scanner .iter ::<$t >() } ;({$e :expr } ) =>{scanner .mscan ($e ) } ;($t :ty ) =>{scanner .scan ::<$t >() } ;}
    let _out = stdout();
    let mut _out = BufWriter::new(_out.lock());
    macro_rules !print {($($arg :tt ) *) =>(::std ::write !(_out ,$($arg ) *) .expect ("io error" ) ) }
    macro_rules !println {($($arg :tt ) *) =>(::std ::writeln !(_out ,$($arg ) *) .expect ("io error" ) ) }
    macro_rules! echo {
        ($iter :expr ) => {
            echo!($iter, '\n')
        };
        ($iter :expr ,$sep :expr ) => {
            let mut iter = $iter.into_iter();
            if let Some(item) = iter.next() {
                print!("{}", item);
            }
            for item in iter {
                print!("{}{}", $sep, item);
            }
            println!();
        };
    }
    let a = scan!({ marker::CharsWithBase('a') });
    let n = a.len();
    let mut p = vec![n];
    let mut st = 0usize;
    const MASK: usize = (1 << 26) - 1;
    for (i, &c) in a.iter().enumerate().rev() {
        st |= 1 << c;
        if st == MASK {
            p.push(i);
            st = 0;
        }
    }
    let seg = SegmentTree::from_vec(
        a.iter().map(|&a| 1usize << a).collect(),
        BitOrOperation::new(),
    );
    let mut l = 0;
    let mut ans = vec![];
    for &r in p.iter().rev() {
        let x = seg.fold(l, r);
        for c in 0..26 {
            if !x & 1 << c != 0 {
                ans.push(c);
                l = r;
                while l < n && a[l] != c {
                    l += 1;
                }
                l += 1;
                break;
            }
        }
    }
    println!(
        "{}",
        ans.iter()
            .map(|&i| (i as u8 + 'a' as u8) as char)
            .collect::<String>()
    );
}