Problem:
Score : 400 points

Problem Statement

N people are arranged in a row from left to right.

You are given a string S of length N consisting of 0 and 1, and a positive integer K.

The i-th person from the left is standing on feet if the i-th character of S is 0, and standing on hands if that character is 1.

You will give the following direction at most K times (possibly zero):

Direction: Choose integers l and r satisfying 1 \leq l \leq r \leq N, and flip the l-th, (l+1)-th, ..., and r-th persons. That is, for each i = l, l+1, ..., r, the i-th person from the left now stands on hands if he/she was standing on feet, and stands on feet if he/she was standing on hands.

Find the maximum possible number of consecutive people standing on hands after at most K directions.

Constraints

N is an integer satisfying 1 \leq N \leq 10^5.

K is an integer satisfying 1 \leq K \leq 10^5.

The length of the string S is N.

Each character of the string S is 0 or 1.

Input

Input is given from Standard Input in the following format:

N K
S

Output

Print the maximum possible number of consecutive people standing on hands after at most K directions.

Sample Input 1

5 1
00010

Sample Output 1

4

We can have four consecutive people standing on hands, which is the maximum result, by giving the following direction:

Give the direction with l = 1, r = 3, which flips the first, second and third persons from the left.

Sample Input 2

14 2
11101010110011

Sample Output 2

8

Sample Input 3

1 1
1

Sample Output 3

1

No directions are necessary.

Solution in Go (1.6) (judge verdict: Accepted)

// https://atcoder.jp/contests/abc124/tasks/abc124_d

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
)

// 0 が連続する個数, 1 が連続する個数, 0 が連続する個数..
func getSuccessiveCounts(N int, chars []rune) []int {
	successiveCounts := []int{}

	successiveCount := 1
	currentChar := chars[0]

	for i := 1; i < N; i++ {
		c := chars[i]

		if c != currentChar {
			successiveCounts = append(successiveCounts, successiveCount)
			successiveCount = 1
			currentChar = c
			continue
		}

		successiveCount++
	}

	successiveCounts = append(successiveCounts, successiveCount)

	return successiveCounts
}

// d: debug IO. it can print debug in test.
func solve(io *Io, d *Io) {
	N := io.NextInt()
	K := io.NextInt()
	S := io.Next()
	chars := []rune(S)

	succCts := getSuccessiveCounts(N, chars)

	accSum := make([]int, len(succCts))
	accSum[0] = succCts[0]

	for i := 1; i < len(succCts); i++ {
		accSum[i] = accSum[i-1] + succCts[i]
	}

	if 2*K >= len(accSum) {
		io.Println(Sum(succCts...))
		return
	}

	max := 0
	firstIsZero := chars[0] == '0'
	for i := 0; 2*K+i <= len(succCts); i++ {
		var partialSum int

		startFromOne := firstIsZero && i%2 == 1 || !firstIsZero && i%2 == 0

		// 1 が先頭のグループを数えれば十分のためスキップ。
		// ただし、文字列の先頭・末尾が 0 であった場合はチェックする必要がある
		if i != 0 && !startFromOne {
			continue
		}

		// 101010000
		if i == 0 && !startFromOne {
			partialSum = accSum[2*K-1]
		} else if i == 0 && startFromOne {
			partialSum = accSum[2*K]
		} else {
			partialSum = accSum[Min(2*K+i, len(succCts)-1)] - accSum[i-1]
		}

		if partialSum > max {
			max = partialSum
		}
	}

	io.Println(max)
}

func main() {
	io := NewIo()
	defer io.Flush()
	solve(io, nil)
}

/* IO Helpers */

// Io combines reader, writer, & tokens as the state when processing the input
type Io struct {
	reader    *bufio.Reader
	writer    *bufio.Writer
	tokens    []string
	nextToken int
}

// NewIo creates Io with Stdin & Stdout
func NewIo() *Io {
	return &Io{
		reader: bufio.NewReader(os.Stdin),
		writer: bufio.NewWriter(os.Stdout),
	}
}

// Flush calls the writer's Flush
func (io *Io) Flush() {
	io.writer.Flush()
}

// NextLine returns the string from reader.ReadLine()
func (io *Io) NextLine() string {
	if io.nextToken < len(io.tokens) {
		panic("io.nextToken < len(io.tokens)")
	}

	var buffer bytes.Buffer

	for {
		line, isPrefix, err := io.reader.ReadLine()
		if err != nil {
			panic(err)
		}
		buffer.Write(line)
		if !isPrefix {
			break
		}
	}

	return buffer.String()
}

// NextLines returns []string from next n lines
func (io *Io) NextLines(n int) []string {
	res := make([]string, n)
	for i := 0; i < n; i++ {
		res[i] = io.NextLine()
	}
	return res
}

// Next returns the string token (partial string of the line divided by spaces)
func (io *Io) Next() string {
	if io.nextToken >= len(io.tokens) {
		line := io.NextLine()
		io.tokens = strings.Fields(line)
		io.nextToken = 0
	}

	res := io.tokens[io.nextToken]
	io.nextToken++
	return res
}

// NextStrings returns the []string from the next n tokens
func (io *Io) NextStrings(n int) []string {
	res := make([]string, n)
	for i := 0; i < n; i++ {
		res[i] = io.Next()
	}
	return res
}

// NextInt returns the int from the next token
func (io *Io) NextInt() int {
	res, err := strconv.Atoi(io.Next())
	if err != nil {
		panic(err)
	}
	return res
}

// NextInts returns the []int from the next n tokens
func (io *Io) NextInts(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = io.NextInt()
	}
	return res
}

// Println prints the input to the writer in an easy-to-see form
func (io *Io) Println(a ...interface{}) {
	var values []string
	for i := 0; i < len(a); i++ {
		values = append(values, "%v")
	}
	io.Printfln(strings.Join(values, " "), a...)
}

// Print calls Fprint to the writer
func (io *Io) Print(a interface{}) {
	fmt.Fprint(io.writer, a)
}

// Printfln calls Fprint to the writer
func (io *Io) Printfln(format string, a ...interface{}) {
	fmt.Fprintf(io.writer, format+"\n", a...)
}

// Debug calls Println and Flush immediately
func (io *Io) Debug(a ...interface{}) {
	if io == nil {
		return
	}

	io.Println(a...)
	io.Flush()
}

/* Math Helpers */

// Max of the ints
func Max(numbers ...int) int {
	max := numbers[0]
	for i, number := range numbers {
		if i == 0 {
			continue
		}
		if number > max {
			max = number
		}
	}
	return max
}

// Min of the ints
func Min(numbers ...int) int {
	max := numbers[0]
	for i, number := range numbers {
		if i == 0 {
			continue
		}
		if max > number {
			max = number
		}
	}
	return max
}

// DigitSum returns sum of the int's digits
func DigitSum(n int) int {
	if n < 0 {
		return -1
	}

	res := 0

	for n > 0 {
		res += n % 10
		n /= 10
	}

	return res
}

// Sum of the ints
func Sum(numbers ...int) int {
	sum := 0

	for _, number := range numbers {
		sum += number
	}

	return sum
}

// Pow calculates the pow for int with O(log e)
func Pow(a, e int) int {
	if a < 0 || e < 0 {
		panic(fmt.Sprintf("Pow was called for a < 0 or e < 0, a: %d, e: %d", a, e))
	}

	if e == 0 {
		return 1
	}

	if e%2 == 0 {
		half := Pow(a, e/2)
		return half * half
	}

	return a * Pow(a, e-1)
}

/* Structure Helpers */

// Set is orderless List
type Set map[interface{}]struct{}

// NewSet returns empty Set
func NewSet() *Set {
	return &Set{}
}

// Add the value to the set
func (set *Set) Add(value interface{}) {
	(*set)[value] = struct{}{}
}

// Includes check the set has the value
func (set *Set) Includes(value interface{}) bool {
	_, included := (*set)[value]
	return included
}

// Remove deletes the value from the set if exists
func (set *Set) Remove(value interface{}) {
	if !set.Includes(value) {
		return
	}

	delete(*set, value)
}

// ImmutableSort returns sorted slice without mutating original one
func ImmutableSort(slice []int) []int {
	res := make([]int, len(slice))
	copy(res, slice)
	sort.Ints(res)
	return res
}

// MinCalculator is helper to get min value
type MinCalculator struct {
	current int
}

// NewMinCalculator returns new MinCalculator with initial value
func NewMinCalculator(initial int) *MinCalculator {
	return &MinCalculator{
		current: initial,
	}
}

// Check compares the value with current and update current if necessary
func (mc *MinCalculator) Check(val int) {
	if val < mc.current {
		mc.current = val
	}
}

// MaxCalculator is helper to get max value
type MaxCalculator struct {
	current int
}

// NewMaxCalculator returns new MaxCalculator with initial value
func NewMaxCalculator(initial int) *MaxCalculator {
	return &MaxCalculator{
		current: initial,
	}
}

// Check compares the value with current and update current if necessary
func (mc *MaxCalculator) Check(val int) {
	if val > mc.current {
		mc.current = val
	}
}

/* Util helpers */

// Ternary is like `cond ? a : b`. should assert the returned value like `Ternary(cond, a, b).(int)`
func Ternary(cond bool, a, b interface{}) interface{} {
	if cond {
		return a
	}

	return b
}

// TernaryInt uses Ternary and assert the value as int
func TernaryInt(cond bool, a, b int) int {
	return Ternary(cond, a, b).(int)
}

// TernaryString uses Ternary and assert the value as string
func TernaryString(cond bool, a, b string) string {
	return Ternary(cond, a, b).(string)
}