Longest Substring of 1's after removing one character
Last Updated :
23 Jul, 2025
Given a binary string S of length N, the task is to find the longest substring consisting of '1's only present in the string after deleting a character from the string.
Examples:
Input: S = "1101"
Output: 3
Explanation:
Removing S[0], S modifies to "101". Longest possible substring of '1's is 1.
Removing S[1], S modifies to "101". Longest possible substring of '1's is 1.
Removing S[2], S modifies to "111". Longest possible substring of '1's is 3.
Removing S[3], S modifies to "110". Longest possible substring of '1's is 2.
Therefore, longest substring of '1's that can be obtained is 3.
Input: S = "011101101"
Output: 5
Method 1: The idea is to traverse the string and search for '0's in the given string. For every character which is found to be '0', add the length of its adjacent substrings of '1'. Print the maximum of all such lengths obtained.
Below is the implementation of the above approach:
C++
// C++ Program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
int longestSubarray(string s)
{
// Add '0' at the end
s += '0';
// Iterator to traverse the string
int i;
// Stores maximum length
// of required substring
int res = 0;
// Stores length of substring of '1'
// preceding the current character
int prev_one = 0;
// Stores length of substring of '1'
// succeeding the current character
int curr_one = 0;
// Counts number of '0's
int numberOfZeros = 0;
// Traverse the string S
for (i = 0; i < s.length(); i++) {
// If current character is '1'
if (s[i] == '1') {
// Increase curr_one by one
curr_one += 1;
}
// Otherwise
else {
// Increment numberofZeros by one
numberOfZeros += 1;
// Count length of substring
// obtained y concatenating
// preceding and succeeding substrings of '1'
prev_one += curr_one;
// Store maximum size in res
res = max(res, prev_one);
// Assign curr_one to prev_one
prev_one = curr_one;
// Reset curr_one
curr_one = 0;
}
}
// If string contains only one '0'
if (numberOfZeros == 1) {
res -= 1;
}
// Return the answer
return res;
}
// Driver Code
int main()
{
string S = "1101";
cout << longestSubarray(S);
return 0;
}
// Java program to implement
// the above approach
import java.util.Arrays;
class GFG{
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
static int longestSubarray(String s)
{
// Add '0' at the end
s += '0';
// Iterator to traverse the string
int i;
// Stores maximum length
// of required substring
int res = 0;
// Stores length of substring of '1'
// preceding the current character
int prev_one = 0;
// Stores length of substring of '1'
// succeeding the current character
int curr_one = 0;
// Counts number of '0's
int numberOfZeros = 0;
// Traverse the string S
for(i = 0; i < s.length(); i++)
{
// If current character is '1'
if (s.charAt(i) == '1')
{
// Increase curr_one by one
curr_one += 1;
}
// Otherwise
else
{
// Increment numberofZeros by one
numberOfZeros += 1;
// Count length of substring
// obtained y concatenating
// preceding and succeeding
// substrings of '1'
prev_one += curr_one;
// Store maximum size in res
res = Math.max(res, prev_one);
// Assign curr_one to prev_one
prev_one = curr_one;
// Reset curr_one
curr_one = 0;
}
}
// If string contains only one '0'
if (numberOfZeros == 1)
{
res -= 1;
}
// Return the answer
return res;
}
// Driver Code
public static void main (String[] args)
{
String S = "1101";
System.out.println(longestSubarray(S));
}
}
// This code is contributed by code_hunt
# Python3 program to implement
# the above approach
# Function to calculate the length of the
# longest substring of '1's that can be
# obtained by deleting one character
def longestSubarray(s):
# Add '0' at the end
s += '0'
# Iterator to traverse the string
i = 0
# Stores maximum length
# of required substring
res = 0
# Stores length of substring of '1'
# preceding the current character
prev_one = 0
# Stores length of substring of '1'
# succeeding the current character
curr_one = 0
# Counts number of '0's
numberOfZeros = 0
# Traverse the string S
for i in range(len(s)):
# If current character is '1'
if (s[i] == '1'):
# Increase curr_one by one
curr_one += 1
# Otherwise
else:
# Increment numberofZeros by one
numberOfZeros += 1
# Count length of substring
# obtained y concatenating
# preceding and succeeding
# substrings of '1'
prev_one += curr_one
# Store maximum size in res
res = max(res, prev_one)
# Assign curr_one to prev_one
prev_one = curr_one
# Reset curr_one
curr_one = 0
# If string contains only one '0'
if (numberOfZeros == 1):
res -= 1
# Return the answer
return res
# Driver Code
if __name__ == '__main__':
S = "1101"
print(longestSubarray(S))
# This code is contributed by ipg2016107
// C# program to implement
// the above approach
using System;
class GFG
{
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
static int longestSubarray(String s)
{
// Add '0' at the end
s += '0';
// Iterator to traverse the string
int i;
// Stores maximum length
// of required substring
int res = 0;
// Stores length of substring of '1'
// preceding the current character
int prev_one = 0;
// Stores length of substring of '1'
// succeeding the current character
int curr_one = 0;
// Counts number of '0's
int numberOfZeros = 0;
// Traverse the string S
for(i = 0; i < s.Length; i++)
{
// If current character is '1'
if (s[i] == '1')
{
// Increase curr_one by one
curr_one += 1;
}
// Otherwise
else
{
// Increment numberofZeros by one
numberOfZeros += 1;
// Count length of substring
// obtained y concatenating
// preceding and succeeding
// substrings of '1'
prev_one += curr_one;
// Store maximum size in res
res = Math.Max(res, prev_one);
// Assign curr_one to prev_one
prev_one = curr_one;
// Reset curr_one
curr_one = 0;
}
}
// If string contains only one '0'
if (numberOfZeros == 1)
{
res -= 1;
}
// Return the answer
return res;
}
// Driver Code
public static void Main(String[] args)
{
String S = "1101";
Console.WriteLine(longestSubarray(S));
}
}
// This code is contributed by shikhasingrajput
<script>
// javascript program to implement
// the above approach
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
function longestSubarray(s)
{
// Add '0' at the end
s += '0';
// Iterator to traverse the string
let i;
// Stores maximum length
// of required substring
let res = 0;
// Stores length of substring of '1'
// preceding the current character
let prev_one = 0;
// Stores length of substring of '1'
// succeeding the current character
let curr_one = 0;
// Counts number of '0's
let numberOfZeros = 0;
// Traverse the string S
for(i = 0; i < s.length; i++)
{
// If current character is '1'
if (s[i] == '1')
{
// Increase curr_one by one
curr_one += 1;
}
// Otherwise
else
{
// Increment numberofZeros by one
numberOfZeros += 1;
// Count length of substring
// obtained y concatenating
// preceding and succeeding
// substrings of '1'
prev_one += curr_one;
// Store maximum size in res
res = Math.max(res, prev_one);
// Assign curr_one to prev_one
prev_one = curr_one;
// Reset curr_one
curr_one = 0;
}
}
// If string contains only one '0'
if (numberOfZeros == 1)
{
res -= 1;
}
// Return the answer
return res;
}
// Driver code
let S = "1101";
document.write(longestSubarray(S));
// This code is contributed by splevel62.
</script>
Time Complexity: O(N)
Auxiliary Space: O(1) as we are not using any extra space.
Method 2: Alternate approach to solve the problem is to use sliding window technique for finding the maximum length of substring containing only '1's after deleting a single character. Follow the steps below to solve the problem:
- Initialize 3 integer variables i, j, with 0 and k with 1
- Iterate over the characters of the string S.
- For every character traversed, check if it is '0' or not. If found to be true, decrement k by 1.
- If k < 0 and character at ith index is '0', increment k and i by one
- Increment j by one.
- Finally, print the length j - i - 1 after complete traversal of the string.
Below is the implementation of the above approach:
C++
// C++ Program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
int longestSubarray(string s)
{
// Initializing i and j as left and
// right boundaries of sliding window
int i = 0, j = 0, k = 1;
for (j = 0; j < s.size(); ++j) {
// If current character is '0'
if (s[j] == '0')
// Decrement k by one
k--;
// If k is less than zero and character
// at ith index is '0'
if (k < 0 && s[i++] == '0')
k++;
}
// Return result
return j - i - 1;
}
// Driver Code
int main()
{
string S = "011101101";
cout << longestSubarray(S);
return 0;
}
// Java Program to implement
// the above approach
import java.util.*;
class GFG{
// Function to calculate the length of the
// longest subString of '1's that can be
// obtained by deleting one character
static int longestSubarray(String s)
{
// Initializing i and j as left and
// right boundaries of sliding window
int i = 0, j = 0, k = 1;
for (j = 0; j < s.length(); ++j)
{
// If current character is '0'
if (s.charAt(j) == '0')
// Decrement k by one
k--;
// If k is less than zero and character
// at ith index is '0'
if (k < 0 && s.charAt(i++) == '0')
k++;
}
// Return result
return j - i - 1;
}
// Driver Code
public static void main(String[] args)
{
String S = "011101101";
System.out.print(longestSubarray(S));
}
}
// This code contributed by gauravrajput1
# Python3 program to implement
# the above approach
# Function to calculate the length of the
# longest substring of '1's that can be
# obtained by deleting one character
def longestSubarray(s):
# Initializing i and j as left and
# right boundaries of sliding window
i = 0
j = 0
k = 1
for j in range(len(s)):
# If current character is '0'
if (s[j] == '0'):
# Decrement k by one
k -= 1
# If k is less than zero and character
# at ith index is '0'
if (k < 0 ):
if s[i] == '0':
k += 1
i += 1
j += 1
# Return result
return j - i - 1
# Driver Code
if __name__ == "__main__" :
S = "011101101"
print(longestSubarray(S))
# This code is contributed by AnkThon
// C# program to implement
// the above approach
using System;
class GFG{
// Function to calculate the length of the
// longest subString of '1's that can be
// obtained by deleting one character
static int longestSubarray(string s)
{
// Initializing i and j as left and
// right boundaries of sliding window
int i = 0, j = 0, k = 1;
for(j = 0; j < s.Length; ++j)
{
// If current character is '0'
if (s[j] == '0')
// Decrement k by one
k -= 1;
// If k is less than zero and character
// at ith index is '0'
if (k < 0 && s[i++] == '0')
k++;
}
// Return result
return j - i - 1;
}
// Driver Code
public static void Main(string[] args)
{
string S = "011101101";
Console.Write(longestSubarray(S));
}
}
// This code is contributed by AnkThon
<script>
// Javascript Program to implement
// the above approach
// Function to calculate the length of the
// longest substring of '1's that can be
// obtained by deleting one character
function longestSubarray(s)
{
// Initializing i and j as left and
// right boundaries of sliding window
var i = 0, j = 0, k = 1;
for (j = 0; j < s.length; ++j) {
// If current character is '0'
if (s[j] == '0')
// Decrement k by one
k--;
// If k is less than zero and character
// at ith index is '0'
if (k < 0 && s[i++] == '0')
k++;
}
// Return result
return j - i - 1;
}
// Driver Code
var S = "011101101";
document.write( longestSubarray(S));
</script>
Time complexity: O(N)
Auxiliary Space: O(1) as we are not using any extra space.
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