Java Solution for Finding the Longest Substring Without Repeating Characters - O(n) Time Complexity

Problem Definition

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

Input: A single string s.

Output: An integer representing the length of the longest substring without repeating characters.

Constraints:

• 0 ≤ s.length ≤ 5 * 104
• The string s consists of English letters, digits, symbols, and spaces.

Example:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Understanding the Problem

The core challenge of this problem is to find the longest substring in a given string that does not contain any repeating characters. This problem is significant in various applications such as data compression, pattern recognition, and text processing. A common pitfall is to overlook the need for an efficient solution, especially given the constraint that the string length can be up to 50,000 characters.

Approach

To solve this problem, we need to consider different approaches:

Naive Solution

The naive solution involves checking all possible substrings and determining if they contain repeating characters. This approach is not optimal due to its high time complexity of O(n³).

Optimized Solution

A more efficient approach is to use the sliding window technique with a hash set to keep track of characters in the current window. This allows us to achieve a linear time complexity of O(n).

Algorithm

Here is a step-by-step breakdown of the optimized algorithm:

1. Initialize two pointers, left and right, both set to the start of the string.
2. Use a hash set to store characters in the current window.
3. Iterate through the string with the right pointer.
4. If the character at right is not in the hash set, add it to the set and update the maximum length.
5. If the character at right is already in the hash set, remove characters from the set starting from left until the duplicate character is removed.
6. Continue this process until the right pointer reaches the end of the string.

Code Implementation

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        // HashSet to store the characters in the current window
        Set<Character> set = new HashSet<>();
        int left = 0, right = 0, maxLength = 0;

        // Iterate through the string with the right pointer
        while (right < s.length()) {
            // If the character at right is not in the set, add it
            if (!set.contains(s.charAt(right))) {
                set.add(s.charAt(right));
                right++;
                maxLength = Math.max(maxLength, right - left);
            } else {
                // If the character at right is in the set, remove the left character
                set.remove(s.charAt(left));
                left++;
            }
        }
        return maxLength;
    }
}

Complexity Analysis

The time complexity of the optimized solution is O(n) because each character is processed at most twice (once by the right pointer and once by the left pointer). The space complexity is O(min(n, m)), where n is the length of the string and m is the size of the character set.

Edge Cases

Potential edge cases include:

• An empty string, which should return 0.
• A string with all identical characters, which should return 1.
• A string with no repeating characters, which should return the length of the string.

Examples:

Input: s = ""
Output: 0

Input: s = "aaaaa"
Output: 1

Input: s = "abcdef"
Output: 6

Testing

To test the solution comprehensively, consider the following test cases:

• Simple cases with short strings.
• Edge cases such as empty strings and strings with all identical characters.
• Complex cases with long strings and varying patterns of repeating characters.

Use testing frameworks such as JUnit to automate the testing process.

Thinking and Problem-Solving Tips

When approaching such problems, consider the following tips:

• Break down the problem into smaller parts and understand each part.
• Think about different approaches and their time complexities.
• Practice solving similar problems to improve your problem-solving skills.

Conclusion

In this blog post, we discussed how to find the longest substring without repeating characters using an optimized approach with a time complexity of O(n). Understanding and solving such problems is crucial for improving your algorithmic skills and preparing for coding interviews.

Additional Resources

For further reading and practice, consider the following resources:

• LeetCode Problem
• GeeksforGeeks Article
• Coursera Algorithms Course