C++ Program To Find Length Of The Longest Substring Without Repeating Characters

Last Updated : 19 Apr, 2023

Given a string str, find the length of the longest substring without repeating characters.

For “ABDEFGABEF”, the longest substring are “BDEFGA” and "DEFGAB", with length 6.
For “BBBB” the longest substring is “B”, with length 1.
For "GEEKSFORGEEKS", there are two longest substrings shown in the below diagrams, with length 7

The desired time complexity is O(n) where n is the length of the string.

Method 1 (Simple : O(n³)): We can consider all substrings one by one and check for each substring whether it contains all unique characters or not. There will be n*(n+1)/2 substrings. Whether a substring contains all unique characters or not can be checked in linear time by scanning it from left to right and keeping a map of visited characters. Time complexity of this solution would be O(n^3).

C++

// C++ program to find the length of the longest substring
// without repeating characters
#include <bits/stdc++.h>
using namespace std;

// This functionr eturns true if all characters in str[i..j]
// are distinct, otherwise returns false
bool areDistinct(string str, int i, int j)
{

    // Note : Default values in visited are false
    vector<bool> visited(26);

    for (int k = i; k <= j; k++) {
        if (visited[str[k] - 'a'] == true)
            return false;
        visited[str[k] - 'a'] = true;
    }
    return true;
}

// Returns length of the longest substring
// with all distinct characters.
int longestUniqueSubsttr(string str)
{
    int n = str.size();
    int res = 0; // result
    for (int i = 0; i < n; i++)
        for (int j = i; j < n; j++)
            if (areDistinct(str, i, j))
                res = max(res, j - i + 1);
    return res;
}

// Driver code
int main()
{
    string str = "geeksforgeeks";
    cout << "The input string is " << str << endl;
    int len = longestUniqueSubsttr(str);
    cout << "The length of the longest non-repeating "
            "character substring is "
         << len;
    return 0;
}

Output

The input string is geeksforgeeks
The length of the longest non-repeating character substring is 7

Method 2 (Better : O(n²)) The idea is to use window sliding. Whenever we see repetition, we remove the previous occurrence and slide the window.

C++

// C++ program to find the length of the longest substring
// without repeating characters
#include <bits/stdc++.h>
using namespace std;

int longestUniqueSubsttr(string str)
{
    int n = str.size();
    int res = 0; // result

    for (int i = 0; i < n; i++) {
        
        // Note : Default values in visited are false
        vector<bool> visited(256);   

        for (int j = i; j < n; j++) {

            // If current character is visited
            // Break the loop
            if (visited[str[j]] == true)
                break;

            // Else update the result if
            // this window is larger, and mark
            // current character as visited.
            else {
                res = max(res, j - i + 1);
                visited[str[j]] = true;
            }
        }

        // Remove the first character of previous
        // window
        visited[str[i]] = false;
    }
    return res;
}

// Driver code
int main()
{
    string str = "geeksforgeeks";
    cout << "The input string is " << str << endl;
    int len = longestUniqueSubsttr(str);
    cout << "The length of the longest non-repeating "
            "character substring is "
         << len;
    return 0;
}

Output

The input string is geeksforgeeks
The length of the longest non-repeating character substring is 7

Method 4 (Linear Time): Let us talk about the linear time solution now. This solution uses extra space to store the last indexes of already visited characters. The idea is to scan the string from left to right, keep track of the maximum length Non-Repeating Character Substring seen so far in res. When we traverse the string, to know the length of current window we need two indexes.
1) Ending index ( j ) : We consider current index as ending index.
2) Starting index ( i ) : It is same as previous window if current character was not present in the previous window. To check if the current character was present in the previous window or not, we store last index of every character in an array lasIndex[]. If lastIndex[str[j]] + 1 is more than previous start, then we updated the start index i. Else we keep same i.

Below is the implementation of the above approach :

C++

// C++ program to find the length of the longest substring
// without repeating characters
#include <bits/stdc++.h>
using namespace std;
#define NO_OF_CHARS 256

int longestUniqueSubsttr(string str)
{
    int n = str.size();

    int res = 0; // result

    // last index of all characters is initialized
    // as -1
    vector<int> lastIndex(NO_OF_CHARS, -1);

    // Initialize start of current window
    int i = 0;

    // Move end of current window
    for (int j = 0; j < n; j++) {

        // Find the last index of str[j]
        // Update i (starting index of current window)
        // as maximum of current value of i and last
        // index plus 1
        i = max(i, lastIndex[str[j]] + 1);

        // Update result if we get a larger window
        res = max(res, j - i + 1);

        // Update last index of j.
        lastIndex[str[j]] = j;
    }
    return res;
}

// Driver code
int main()
{
    string str = "geeksforgeeks";
    cout << "The input string is " << str << endl;
    int len = longestUniqueSubsttr(str);
    cout << "The length of the longest non-repeating "
            "character substring is "
         << len;
    return 0;
}

Output

The input string is geeksforgeeks
The length of the longest non-repeating character substring is 7

Time Complexity: O(n + d) where n is length of the input string and d is number of characters in input string alphabet. For example, if string consists of lowercase English characters then value of d is 26.
Auxiliary Space: O(d)

Alternate Implementation :

C++

#include <bits/stdc++.h>
using namespace std;

int longestUniqueSubsttr(string s)
{
    
    // Creating a set to store the last positions 
    // of occurrence
    map<char, int> seen ;
    int maximum_length = 0;

    // Starting the initial point of window to index 0
    int start = 0;

    for(int end = 0; end < s.length(); end++) 
    {
        
        // Checking if we have already seen the element or
        // not
        if (seen.find(s[end]) != seen.end())
        {
            
            // If we have seen the number, move the start
            // pointer to position after the last occurrence
            start = max(start, seen[s[end]] + 1);
        }

        // Updating the last seen value of the character
        seen[s[end]] = end;
        maximum_length = max(maximum_length, 
                             end - start + 1);
    }
    return maximum_length;
}

// Driver code
int main()
{
    string s = "geeksforgeeks";
    cout << "The input String is " << s << endl;
    int length = longestUniqueSubsttr(s);
    
    cout<<"The length of the longest non-repeating character "
        <<"substring is "
        << length;
}

// This code is contributed by ukasp

Output

The input String is geeksforgeeks
The length of the longest non-repeating character substring is 7

As an exercise, try the modified version of the above problem where you need to print the maximum length NRCS also (the above program only prints the length of it).

Please refer complete article on Length of the longest substring without repeating characters for more details!