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Longest subsegment of ‘1’s formed by changing at most k ‘0’s

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  • Difficulty Level : Medium
  • Last Updated : 29 Jul, 2022
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Given a binary array a[] and a number k, we need to find length of the longest subsegment of ‘1’s possible by changing at most k ‘0’s. 
Examples: 
 

Input : a[] = {1, 0, 0, 1, 1, 0, 1}, 
          k = 1.
Output : 4
Explanation : Here, we should only change 1
zero(0). Maximum possible length we can get
is by changing the 3rd zero in the array, 
we get a[] = {1, 0, 0, 1, 1, 1, 1}

Input : a[] = {1, 0, 0, 1, 0, 1, 0, 1, 0, 1}, 
         k = 2.
Output : 5
Output: Here, we can change only 2 zeros. 
Maximum possible length we can get is by 
changing the 3rd and 4th (or) 4th and 5th 
zeros.

 

We can solve this problem using two pointers technique. Let us take a subarray [l, r] which contains at most k zeroes. Let our left pointer be l and right pointer be r. We always maintain our subsegment [l, r] to contain no more than k zeroes by moving the left pointer l. Check at every step for maximum size (i.e, r-l+1). 
 

C++




// CPP program to find length of longest
// subsegment of all 1's by changing at
// most k 0's
#include <iostream>
using namespace std;
 
int longestSubSeg(int a[], int n, int k)
{
    int cnt0 = 0;
    int l = 0;
    int max_len = 0;
 
    // i decides current ending point
    for (int i = 0; i < n; i++) {
        if (a[i] == 0)
            cnt0++;
 
        // If there are more 0's move
        // left point for current ending
        // point.
        while (cnt0 > k) {
            if (a[l] == 0)
                cnt0--;
            l++;
        }
 
        max_len = max(max_len, i - l + 1);
    }
 
    return max_len;
}
 
// Driver code
int main()
{
    int a[] = { 1, 0, 0, 1, 0, 1, 0, 1 };
    int k = 2;
    int n = sizeof(a) / sizeof(a[0]);
    cout << longestSubSeg(a, n, k);
    return 0;
}

Java




// Java program to find length of
// longest subsegment of all 1's
// by changing at most k 0's
import java.io.*;
 
class GFG {
 
static int longestSubSeg(int a[], int n,
                                  int k)
{
    int cnt0 = 0;
    int l = 0;
    int max_len = 0;
 
    // i decides current ending point
    for (int i = 0; i < n; i++) {
        if (a[i] == 0)
            cnt0++;
 
        // If there are more 0's move
        // left point for current ending
        // point.
        while (cnt0 > k) {
            if (a[l] == 0)
                cnt0--;
            l++;
        }
 
        max_len = Math.max(max_len, i - l + 1);
    }
 
    return max_len;
}
 
// Driver code
public static void main (String[] args)
{
    int a[] = { 1, 0, 0, 1, 0, 1, 0, 1 };
    int k = 2;
    int n = a.length;
    System.out.println( longestSubSeg(a, n, k));
         
}
}
 
// This code is contributed by vt_m

Python3




# Python3 program to find length
# of longest subsegment of all 1's 
# by changing at most k 0's
 
def longestSubSeg(a, n, k):
 
    cnt0 = 0
    l = 0
    max_len = 0;
 
    # i decides current ending point
    for i in range(0, n):
        if a[i] == 0:
            cnt0 += 1
 
        # If there are more 0's move
        # left point for current
        # ending point.
        while (cnt0 > k):
            if a[l] == 0:
                cnt0 -= 1
            l += 1
         
 
        max_len = max(max_len, i - l + 1);
     
 
    return max_len
 
# Driver code
a = [1, 0, 0, 1, 0, 1, 0, 1 ]
k = 2
n = len(a)
print(longestSubSeg(a, n, k))
 
# This code is contributed by Smitha Dinesh Semwal

C#




// C# program to find length of
// longest subsegment of all 1's
// by changing at most k 0's
using System;
 
class GFG {
 
    static int longestSubSeg(int[] a, int n,
                                      int k)
    {
        int cnt0 = 0;
        int l = 0;
        int max_len = 0;
 
        // i decides current ending point
        for (int i = 0; i < n; i++)
        {
            if (a[i] == 0)
                cnt0++;
 
            // If there are more 0's move
            // left point for current ending
            // point.
            while (cnt0 > k) {
                if (a[l] == 0)
                    cnt0--;
                l++;
            }
 
            max_len = Math.Max(max_len, i - l + 1);
        }
 
        return max_len;
    }
 
    // Driver code
    public static void Main()
    {
        int[] a = { 1, 0, 0, 1, 0, 1, 0, 1 };
        int k = 2;
        int n = a.Length;
        Console.WriteLine(longestSubSeg(a, n, k));
    }
}
 
// This code is contributed by vt_m

PHP




<?php
// PHP program to find length of longest
// subsegment of all 1's by changing at
// most k 0's
 
function longestSubSeg( $a, $n, $k)
{
    $cnt0 = 0;
    $l = 0;
    $max_len = 0;
 
    // i decides current ending point
    for($i = 0; $i < $n; $i++)
    {
        if ($a[$i] == 0)
            $cnt0++;
 
        // If there are more 0's move
        // left point for current ending
        // point.
        while ($cnt0 > $k)
        {
            if ($a[$l] == 0)
                $cnt0--;
            $l++;
        }
 
        $max_len = max($max_len, $i - $l + 1);
    }
 
    return $max_len;
}
 
    // Driver code
    $a = array(1, 0, 0, 1, 0, 1, 0, 1);
    $k = 2;
    $n = count($a);
    echo longestSubSeg($a, $n, $k);
 
// This code is contributed by anuj_67.
?>

Javascript




<script>
 
    // JavaScript program to find length of
    // longest subsegment of all 1's
    // by changing at most k 0's
     
    function longestSubSeg(a, n, k)
    {
        let cnt0 = 0;
        let l = 0;
        let max_len = 0;
   
        // i decides current ending point
        for (let i = 0; i < n; i++)
        {
            if (a[i] == 0)
                cnt0++;
   
            // If there are more 0's move
            // left point for current ending
            // point.
            while (cnt0 > k) {
                if (a[l] == 0)
                    cnt0--;
                l++;
            }
   
            max_len = Math.max(max_len, i - l + 1);
        }
   
        return max_len;
    }
     
    let a = [ 1, 0, 0, 1, 0, 1, 0, 1 ];
    let k = 2;
    let n = a.length;
    document.write(longestSubSeg(a, n, k));
     
</script>

Output

5

There is another O(n) approach, where we can keep a track of number of 1s will be discarded if a 0 is not converted to one.

As we move right and encounter more 0s that what is permitted, we reduce one 0 to the left and at that time we reduce the count of ones by the 1s adjacent to the left-most zero being discarded.

Java




/*package whatever //do not write package name here */
 
import java.io.*;
 
import java.util.ArrayList;
import java.util.List;
 
public class Solution {
        public int solve(int[] binaryList, int conversionLimit) {
 
            List<Integer> listOfOnesCovered = new ArrayList<>();
 
            int oneCount = 0;
            for (int number : binaryList) {
                if (number == 0) {
                    listOfOnesCovered.add(oneCount > 0 ? oneCount + 1 : 1);
                    oneCount = 0;
                } else {
                    ++oneCount;
                }
            }
 
            int totalOnes = 0;
            int maxOnes = 0;
            int zeroCount = 0;
 
            for (int integer : binaryList) {
                ++totalOnes;
                if (integer == 0) {
 
                    if (zeroCount >= conversionLimit) {
                        totalOnes -= listOfOnesCovered.get(zeroCount - conversionLimit);
                    }
                    ++zeroCount;
                }
                maxOnes = Math.max(totalOnes, maxOnes);
            }
 
            return maxOnes;
        }
 
        public static void main (String[] args){
            System.out.println(
                new Solution().solve(new int[]{1, 0, 0, 0, 0,1,1, 1}, 2)
            );
 
        }
    }

C#




// C# code for the above approach
 
using System;
using System.Collections;
 
public class Solution {
 
    public int solve(int[] binaryList, int conversionLimit)
    {
        ArrayList listOfOnesCovered = new ArrayList();
        int oneCount = 0;
        foreach(int number in binaryList)
        {
            if (number == 0) {
                listOfOnesCovered.Add(
                    oneCount > 0 ? oneCount + 1 : 1);
                oneCount = 0;
            }
            else {
                ++oneCount;
            }
        }
        int totalOnes = 0;
        int maxOnes = 0;
        int zeroCount = 0;
 
        foreach(int integer in binaryList)
        {
            ++totalOnes;
            if (integer == 0) {
                if (zeroCount >= conversionLimit) {
                    totalOnes -= (int)listOfOnesCovered
                        [zeroCount - conversionLimit];
                }
                ++zeroCount;
            }
            maxOnes = Math.Max(totalOnes, maxOnes);
        }
 
        return maxOnes;
    }
 
    static public void Main()
    {
 
        // Code
        Solution s = new Solution();
        Console.WriteLine(s.solve(
            new int[] { 1, 0, 0, 0, 0, 1, 1, 1 }, 2));
    }
}
 
// This code is contributed by lokeshmvs21.

Output

5

 


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