Problem:
Swapping Characters

You are given a string and a number k. You are suggested to generate new strings by swapping any adjacent pair of characters in the string up to k times. Write a program to report the lexicographically smallest string among them.

Input

The input is given in the following format.

s
k

The first line provides a string s. The second line provides the maximum number of swapping operations k (0 ≤ k ≤ 109). The string consists solely of lower-case alphabetical letters and has a length between 1 and 2 × 105.

Output

Output the lexicographically smallest string.

Sample Input 1

pckoshien
3

Sample Output 1

ckopshien

Sample Input 2

pckoshien
10

Sample Output 2

cekophsin

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Threading;
using System.Runtime.CompilerServices;
using System.Text;
using System.Diagnostics;
using System.Numerics;
using static System.Console;
using static System.Convert;
using static System.Math;
using static Template;

class Solver
{
    public Scanner sc;
    public void Solve()
    {
        var S = sc.Str;
        var K = sc.Int;
        var Q = Create(26, () => new Queue<int>());
        var set = new Set<int>();
        for (int i = 0; i < S.Length; i++)
        {
            set.Add(i);
            Q[S[i] - 'a'].Enqueue(i);
        }
        var res = new char[S.Length];
        for(int i = 0; i < S.Length; i++)
        {
            var cost = Create(26, () => int.MaxValue / 2);
            for(int j = 0; j < 26; j++)
            {
                if (!Q[j].Any()) continue;
                cost[j] = set.LowerBound(Q[j].Peek());
            }
            for(int j=0; j < 26; j++)
            {
                if (K >= cost[j]) { K -= cost[j]; res[i] = (char)(j + 'a');set.Remove(Q[j].Dequeue());break; }
            }
        }
        Console.WriteLine(new string(res));
    }
}

public class Set<T>
{
    Node root;
    readonly IComparer<T> comparer;
    readonly Node nil;
    public bool IsMultiSet { get; set; }
    public Set(IComparer<T> comparer)
    {
        nil = new Node(default(T));
        root = nil;
        this.comparer = comparer;
    }
    public Set(Comparison<T> comaprison) : this(Comparer<T>.Create(comaprison)) { }
    public Set() : this(Comparer<T>.Default) { }
    public bool Add(T v)
    {
        return insert(ref root, v);
    }
    public bool Remove(T v)
    {
        return remove(ref root, v);
    }
    public T this[int index] { get { return find(root, index); } }
    public int Count { get { return root.Count; } }
    public void RemoveAt(int k)
    {
        if (k < 0 || k >= root.Count) throw new ArgumentOutOfRangeException();
        removeAt(ref root, k);
    }
    public T[] Items
    {
        get
        {
            var ret = new T[root.Count];
            var k = 0;
            walk(root, ret, ref k);
            return ret;
        }
    }
    void walk(Node t, T[] a, ref int k)
    {
        if (t.Count == 0) return;
        walk(t.lst, a, ref k);
        a[k++] = t.Key;
        walk(t.rst, a, ref k);
    }

    bool insert(ref Node t, T key)
    {
        if (t.Count == 0) { t = new Node(key); t.lst = t.rst = nil; t.Update(); return true; }
        var cmp = comparer.Compare(t.Key, key);
        bool res;
        if (cmp > 0)
            res = insert(ref t.lst, key);
        else if (cmp == 0)
        {
            if (IsMultiSet) res = insert(ref t.lst, key);
            else return false;
        }
        else res = insert(ref t.rst, key);
        balance(ref t);
        return res;
    }
    bool remove(ref Node t, T key)
    {
        if (t.Count == 0) return false;
        var cmp = comparer.Compare(key, t.Key);
        bool ret;
        if (cmp < 0) ret = remove(ref t.lst, key);
        else if (cmp > 0) ret = remove(ref t.rst, key);
        else
        {
            ret = true;
            var k = t.lst.Count;
            if (k == 0) { t = t.rst; return true; }
            if (t.rst.Count == 0) { t = t.lst; return true; }


            t.Key = find(t.lst, k - 1);
            removeAt(ref t.lst, k - 1);
        }
        balance(ref t);
        return ret;
    }
    void removeAt(ref Node t, int k)
    {
        var cnt = t.lst.Count;
        if (cnt < k) removeAt(ref t.rst, k - cnt - 1);
        else if (cnt > k) removeAt(ref t.lst, k);
        else
        {
            if (cnt == 0) { t = t.rst; return; }
            if (t.rst.Count == 0) { t = t.lst; return; }

            t.Key = find(t.lst, k - 1);
            removeAt(ref t.lst, k - 1);
        }
        balance(ref t);
    }
    void balance(ref Node t)
    {
        var balance = t.lst.Height - t.rst.Height;
        if (balance == -2)
        {
            if (t.rst.lst.Height - t.rst.rst.Height > 0) { rotR(ref t.rst); }
            rotL(ref t);
        }
        else if (balance == 2)
        {
            if (t.lst.lst.Height - t.lst.rst.Height < 0) rotL(ref t.lst);
            rotR(ref t);
        }
        else t.Update();
    }

    T find(Node t, int k)
    {
        if (k < 0 || k > root.Count) throw new ArgumentOutOfRangeException();
        for (; ; )
        {
            if (k == t.lst.Count) return t.Key;
            else if (k < t.lst.Count) t = t.lst;
            else { k -= t.lst.Count + 1; t = t.rst; }
        }
    }
    public int LowerBound(T v)
    {
        var k = 0;
        var t = root;
        for (; ; )
        {
            if (t.Count == 0) return k;
            if (comparer.Compare(v, t.Key) <= 0) t = t.lst;
            else { k += t.lst.Count + 1; t = t.rst; }
        }
    }
    public int UpperBound(T v)
    {
        var k = 0;
        var t = root;
        for (; ; )
        {
            if (t.Count == 0) return k;
            if (comparer.Compare(t.Key, v) <= 0) { k += t.lst.Count + 1; t = t.rst; }
            else t = t.lst;
        }
    }
    public bool Contains(T v)
    {
        var k = 0;
        var t = root;
        for (; ; )
        {
            if (t.Count == 0) return false;
            if (comparer.Compare(t.Key, v) == 0) return true;
            if (comparer.Compare(t.Key, v) < 0) { k += t.lst.Count + 1; t = t.rst; }
            else t = t.lst;
        }
    }
    void rotR(ref Node t)
    {
        var l = t.lst;
        t.lst = l.rst;
        l.rst = t;
        t.Update();
        l.Update();
        t = l;
    }
    void rotL(ref Node t)
    {
        var r = t.rst;
        t.rst = r.lst;
        r.lst = t;
        t.Update();
        r.Update();
        t = r;
    }
    public T[] ToArray()
    {
        var rt = new T[Count]; var idx = 0;
        Action<Node> dfs = null;
        dfs = a =>
        {
            if (a.lst != nil) dfs(a.lst);
            rt[idx++] = a.Key;
            if (a.rst != nil) dfs(a.rst);
        };
        dfs(root);
        return rt;
    }

    class Node
    {
        public Node(T key)
        {
            Key = key;
        }
        public int Count { get; private set; }
        public sbyte Height { get; private set; }
        public T Key { get; set; }
        public Node lst, rst;
        public void Update()
        {
            Count = 1 + lst.Count + rst.Count;
            Height = (sbyte)(1 + Math.Max(lst.Height, rst.Height));
        }
        public override string ToString()
        {
            return string.Format("Count = {0}, Key = {1}", Count, Key);
        }
    }
}
#region Template
public static class Template
{
    static void Main(string[] args)
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        var sol = new Solver() { sc = new Scanner() };
        //var th = new Thread(sol.Solve, 1 << 26);th.Start();th.Join();
        sol.Solve();
        Console.Out.Flush();
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmin<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) > 0) { a = b; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmax<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) < 0) { a = b; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(ref T a, ref T b) { var t = b; b = a; a = t; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(this IList<T> A, int i, int j) { var t = A[i]; A[i] = A[j]; A[j] = t; }
    public static T[] Create<T>(int n, Func<T> f) { var rt = new T[n]; for (var i = 0; i < rt.Length; ++i) rt[i] = f(); return rt; }
    public static T[] Create<T>(int n, Func<int, T> f) { var rt = new T[n]; for (var i = 0; i < rt.Length; ++i) rt[i] = f(i); return rt; }
    public static string Concat<T>(this IEnumerable<T> A, string sp) => string.Join(sp, A);
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> A) => A.OrderBy(v => Guid.NewGuid());
    public static int CompareTo<T>(this T[] A, T[] B, Comparison<T> cmp = null) { cmp = cmp ?? Comparer<T>.Default.Compare; for (var i = 0; i < Min(A.Length, B.Length); i++) { int c = cmp(A[i], B[i]); if (c > 0) return 1; else if (c < 0) return -1; } if (A.Length == B.Length) return 0; if (A.Length > B.Length) return 1; else return -1; }
    public static int ArgMax<T>(this IList<T> A, Comparison<T> cmp = null) { cmp = cmp ?? Comparer<T>.Default.Compare; T max = A[0]; int rt = 0; for (int i = 1; i < A.Count; i++) if (cmp(max, A[i]) < 0) { max = A[i]; rt = i; } return rt; }
    public static T PopBack<T>(this List<T> A) { var v = A[A.Count - 1]; A.RemoveAt(A.Count - 1); return v; }
    public static void Fail<T>(T s) { Console.WriteLine(s); Console.Out.Close(); Environment.Exit(0); }
}
public class Scanner
{
    public string Str => Console.ReadLine().Trim();
    public int Int => int.Parse(Str);
    public long Long => long.Parse(Str);
    public double Double => double.Parse(Str);
    public int[] ArrInt => Str.Split(' ').Select(int.Parse).ToArray();
    public long[] ArrLong => Str.Split(' ').Select(long.Parse).ToArray();
    public char[][] Grid(int n) => Create(n, () => Str.ToCharArray());
    public int[] ArrInt1D(int n) => Create(n, () => Int);
    public long[] ArrLong1D(int n) => Create(n, () => Long);
    public int[][] ArrInt2D(int n) => Create(n, () => ArrInt);
    public long[][] ArrLong2D(int n) => Create(n, () => ArrLong);
    private Queue<string> q = new Queue<string>();
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public T Next<T>() { if (q.Count == 0) foreach (var item in Str.Split(' ')) q.Enqueue(item); return (T)Convert.ChangeType(q.Dequeue(), typeof(T)); }
    public void Make<T1>(out T1 v1) => v1 = Next<T1>();
    public void Make<T1, T2>(out T1 v1, out T2 v2) { v1 = Next<T1>(); v2 = Next<T2>(); }
    public void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3) { Make(out v1, out v2); v3 = Next<T3>(); }
    public void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4) { Make(out v1, out v2, out v3); v4 = Next<T4>(); }
    public void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5) { Make(out v1, out v2, out v3, out v4); v5 = Next<T5>(); }
    public void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6) { Make(out v1, out v2, out v3, out v4, out v5); v6 = Next<T6>(); }
    public void Make<T1, T2, T3, T4, T5, T6, T7>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6, out T7 v7) { Make(out v1, out v2, out v3, out v4, out v5, out v6); v7 = Next<T7>(); }
    //public (T1, T2) Make<T1, T2>() { Make(out T1 v1, out T2 v2); return (v1, v2); }
    //public (T1, T2, T3) Make<T1, T2, T3>() { Make(out T1 v1, out T2 v2, out T3 v3); return (v1, v2, v3); }
    //public (T1, T2, T3, T4) Make<T1, T2, T3, T4>() { Make(out T1 v1, out T2 v2, out T3 v3, out T4 v4); return (v1, v2, v3, v4); }
}

#endregion