**Difficulty:** Medium, **Asked-in:** Microsoft, Amazon, Morgan Stanley

**Key takeaway:** An excellent problem to learn time complexity optimization using sliding window approach. We can solve a lot of problems using a similar idea.

Write a program to find the length of the longest substring without repeating characters.

- A substring is a continuous sub-part of a string. We aim to determine the maximum substring that has all unique characters.
- The input string consists of English letters, digits, and symbols.
- The answer must be a continuous substring, not a subsequence of the string.
- There can be more than one longest substring without repeating characters of equal length. Therefore, we only need to return the length of the longest substring.

Input: str = "abcbbcab", Output: 3

Explanation: There are three substrings of unique characters with the longest length of 3: "abc", "bca", and "cab". So the output is 3.

Input: str = "bbbbb", Output: 1

Explanation: There is only one unique character, so the output is 1.

Input: str = "enjoyalgorithms", Output: 11

Explanation: The only repeating character is "o", and the answer is "yalgorithms", which has a length of 11.

- Brute force approach using two nested loops
- Using sliding window approach
- Optimized sliding window approach

This is a simple approach where the idea is to explore all substrings, and for each substring, we check whether it contains unique characters or not. For a string with n characters, there will be n*(n + 1)/2 substrings.

**Step 1:** We initialize the variable **maxLength** to keep track of the longest substring with unique characters: maxLength = 0.

**Step 2:** Now, we explore all possible substrings using two nested loops: an outer loop from i = 0 to n - 1, and an inner loop from j = i to n - 1. The idea is to pick a starting character via the outer loop and explore all substrings starting from that character using the inner loop.

- For each substring, str[i, j], i.e., starting from index i and ending at index j, we use the function
**checkUniqueSubstring(str, i, j)**to check if all characters in the substring are unique or not. It will return true if all characters are unique; otherwise, it will return false. - If the function
**checkUniqueSubstring(str, i, j)**returns true, we compare maxLength with the length of the substring. If**maxLength < j - i + 1**, we update the variable maxLength.

**Step 3:** By the end of the nested loops, we return the value stored in maxLength.

- To check whether a substring has duplicate characters or not, we use a set
**visited[256]**to store the status of each possible character in the substring, i.e., either true or false. - We go through each character from index i to j using a loop. For each character, we check if it is already present in the set visited[] or not. If it is present, we return
**false**. Otherwise, we update the status of that character as**true**in the set visited[]. - By the end of the loop, we return true, i.e., the characters are unique in the substring.

```
bool checkUniqueSubstring(string str, int i, int j)
{
bool visited[256] = {false};
for (int k = i; k <= j; k = k + 1)
{
if (visited[str[k]] == true)
return false;
visited[str[k]] = true;
}
return true;
}
int longestSubstring(string str, int n)
{
int maxLength = 0;
for (int i = 0; i < n; i = i + 1)
{
for (int j = i; j < n; j = j + 1)
{
if (checkUniqueSubstring(str, i, j) == true)
maxLength = max(maxLength, j - i + 1);
}
}
return maxLength;
}
```

```
def checkUniqueSubstring(str, i, j):
visited = [False] * 256
for k in range(i, j+1):
if visited[ord(str[k])]:
return False
visited[ord(str[k])] = True
return True
def longestSubstring(str, n):
maxLength = 0
for i in range(n):
for j in range(i, n):
if checkUniqueSubstring(str, i, j):
maxLength = max(maxLength, j - i + 1)
return maxLength
```

```
class Solution
{
private boolean checkUniqueSubstring(String str, int i, int j)
{
boolean[] visited = new boolean[256];
for (int k = i; k <= j; k = k + 1)
{
if (visited[str.charAt(k)])
return false;
visited[str.charAt(k)] = true;
}
return true;
}
public int longestSubstring(String str, int n)
{
int maxLength = 0;
for (int i = 0; i < n; i = i + 1)
{
for (int j = i; j < n; j = j + 1)
{
if (checkUniqueSubstring(str, i, j))
maxLength = Math.max(maxLength, j - i + 1);
}
}
return maxLength;
}
}
```

We are exploring all n(n + 1)/2 substrings using nested loops. For each substring, function **areUnique(str, i, j)** takes O(j - i + 1) time to check whether the substring str[i, j] has unique characters or not.

Time complexity = n(n + 1)/2 * O( j - i + 1) = O(n^2) * O( j - i + 1). For the upper bound of the worst-case analysis, we consider O(j - i +1) as O(n). Why? Think! For a better understanding of loop analysis, refer to the analysis of loop blog. So, the overall time complexity in the worst case = O(n^2) * O(n) = O(n^3).

We are using a few extra variables and a constant-size set visited[]. Therefore, the space complexity is O(1).

Now a critical question is: How can we optimize the time complexity of the brute force approach? Think! The idea would be to apply sliding window approach and track repetition of characters in each substring, i.e., whenever we found repetition in the current window, we slide the window and move to the next substring.

Here is an optimization insight: In brute force idea, we repeatedly check a substring starting from character str[i] to see if it has duplicate characters or not. We can optimize it further because, during the loop, if substring str[i, j-1] is already checked to have no duplicate characters, then for substring str[i, j], we only need to check if str[j] is already present in substring str[i, j-1] or not.

We can use a hash set or extra array to check whether character str[j] is there in substring str[i, j-1] or not, which can be done in O(1) time complexity. How do we implement this? Let's think!

**Note:** Sliding window is an abstract concept commonly used in array/string problems. A window is a range of elements in the array/string defined by the start and end indices, where the window "slides" in a forward direction by incrementing the left or right end.

**Step 1:** We initialize the variable **maxLength** to keep track of the longest substring with unique characters i.e. maxLength = 0.

**Step 2:** We run an outer loop until i < n to explore the substring window starting from character str[i].

**Step 3:** Inside the loop, we initialize the right end of the window i.e., **j = i**. We also use an array **visited[256]** as a hash set to store the status of characters in the current window [i, j). Initially, **i = j = 0**.

**Step 4:** We run an inner loop to check each substring in the window [i, j). This loop will run until **j < n && visited[str[j]] == false**.

- If
**str[j]**is not present in the window [i, j) i.e., if**visited[str[j]] == false**, we mark**str[j]**in**visited[]**and keep track of the longest substring length. Now we slide the current window to one right by incrementing**j**and continue the same process in the inner loop until**str[j]**is not present in the window [i, j). - If the character
**str[j]**is already present or visited in the current window, we increment**i**by 1 and go to the next iteration of the outer loop. Now we check the new window of the substring starting from index**i + 1**. Note: Before moving to the next iteration of the outer loop, we should set the status of the character**str[i]**in the set**visited[]**i.e.,**visited[str[i]] = false**. (Think!)

**Step 5:** The outer loop will stop when the left end of the sliding window reaches the end of the string i.e., **i = n**. At this stage, we return the value stored in the variable **maxLength**.

```
int longestSubstring(string str, int n)
{
int maxLength = 0;
int i = 0;
while (i < n)
{
bool visited[256] = {false};
int j = i;
while (j < n && visited[str[j]] == false)
{
maxLength = max(maxLength, j - i + 1);
visited[str[j]] = true;
j = j + 1;
}
visited[str[i]] = false;
i = i + 1;
}
return maxLength;
}
```

```
def longestSubstring(str, n):
maxLength = 0
i = 0
while i < n:
visited = [False] * 256
j = i
while j < n and not visited[ord(str[j])]:
maxLength = max(maxLength, j - i + 1)
visited[ord(str[j])] = True
j = j + 1
visited[ord(str[i])] = False
i = i + 1
return maxLength
```

```
class Solution
{
public int longestSubstring(String str, int n)
{
int maxLength = 0;
int i = 0;
while (i < n)
{
boolean[] visited = new boolean[256];
int j = i;
while (j < n && !visited[str.charAt(j)])
{
maxLength = Math.max(maxLength, j - i + 1);
visited[str.charAt(j)] = true;
j = j + 1;
}
visited[str.charAt(i)] = false;
i = i + 1;
}
return maxLength;
}
}
```

We are using nested loops and performing an O(1) operation at each iteration. The worst-case occur when inner loop runs till the end every time. Or in other words, worst-case will occur when all characters in string are unique. Think! In such situation, nested loop will explore all possible substrings i.e. n(n + 1)/2. Time complexity = n(n + 1)/2 * O(1) = O(n^2).

We are using constant size set visited[] of size 256, so space complexity = O(1).

Now, the critical questions are: Can we optimize the above approach further and solve this problem in O(n) time complexity? Can we think of removing the inner loop of the previous approach? Here is a hint:

- At each iteration of the outer loop in the previous approach, we ignore the previous window [i, j) and reset j = i to start a new window. In other words, we are starting with a new window size of 1 at each iteration of the outer loop.
- If the characters in window [i, j) are unique, then the characters in window [i + 1, j) will also be unique. So, in the next iteration of the outer loop, there is no need to reset j = i (Think!).

- We scan the string from left to right using two window pointers, i and j. We also keep track of the longest length of the substring using the variable maxLength and maintain a visited[256] table to keep track of visited characters. Note: All values in the visited[] table are initially false.
- If visited[str[j]] is false, we set visited[str[j]] = true and slide the current window to the right by incrementing the pointer j. We also update the longest length of the substring with unique characters, i.e., maxLength = max(maxLength, j - i).
- If the character str[j] is already marked true in the visited[] table, we have found a repeated character in the current substring starting from index i. So we set visited[str[j]] = false and slide the window to the right by incrementing the pointer i.
- This process will end when either pointer reaches the end of the string, i.e., i = n or j = n. At this stage, we return the value stored in the variable maxLength.

```
int longestSubstring(string str, int n)
{
if (n == 0)
return 0;
bool visited[256] = {false};
int maxLength = 0;
int i = 0, j = 0;
while (i < n && j < n)
{
if (visited[str[j]] == false)
{
visited[str[j]] = true;
j = j + 1;
maxLength = max(maxLength, j - i);
}
else
{
visited[str[i]] = false;
i = i + 1;
}
}
return maxLength;
}
```

```
def longestSubstring(str, n):
if n == 0:
return 0
visited = [False] * 256
maxLength = 0
i = 0
j = 0
while i < n and j < n:
if not visited[ord(str[j])]:
visited[ord(str[j])] = True
j = j + 1
maxLength = max(maxLength, j - i)
else:
visited[ord(str[i])] = False
i = i + 1
return maxLength
```

```
class Solution
{
public int longestSubstring(String str, int n)
{
if (n == 0)
return 0;
boolean[] visited = new boolean[256];
int maxLength = 0;
int i = 0;
int j = 0;
while (i < n && j < n)
{
if (visited[str.charAt(j)] == false)
{
visited[str.charAt(j)] = true;
j = j + 1;
maxLength = Math.max(maxLength, j - i);
}
else
{
visited[str.charAt(i)] = false;
i = i + 1;
}
}
return maxLength;
}
}
```

At each iteration of the while loop, we either increment pointer i or pointer j. So, the loop will run n times and perform a constant number of operations at each iteration. The time complexity is O(n).

We are using a constant-size set visited[] of size 256, so the space complexity is O(1).

- Can we optimize the efficient approach further?
- Instead of using the visited[256] table, can we use a hash table to store the status of each character in a substring window?
- Can we think of solving the problem using some other approach?
- How can we modify the above approaches to print all the longest substrings in lexicographic order?
- Explore patterns of problem-solving using the sliding window technique.

- Longest substring with at most two distinct characters
- Longest substring with at most K distinct characters
- Find maximum consecutive 1s in an array
- Max continuous series of 1s
- Count distinct elements in every window
- Sliding window maximum
- Subarrays with K Different Integers

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