Longest Substring without repeating characters - Leet Code Solution

August 10, 2019

Problem Statement

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

Example 1:
Input: "abcabcbb"
Output: 3 

Example 2:
Input: "bbbbb"
Output: 1

Example 3:
Input: "pwwkew"
Output: 3

1. Brute Force Solution

As the question demands substring. Why not we find out what are the substrings out of the input strings first. Lookout for the steps:

Get all unique substrings. By unique I meant, there might be duplicate substrings as well. Count only unique one.
Iterate over all substrings, and proceed for substring which have all unique characters
Get the length of string
Keep track of maximum length string

You got the answer. Lets look at the code:

public class SimpleSolution {
    private String str;
    public SimpleSolution(String str) {
       this.str = str;
    }

    private Set<String> getAllUniqueSubstrings() {
       Set<String> res = new HashSet<>();

       int l = this.str.length();
       for (int i=0; i<l; i++) {
          for (int j=i+1; j<l; j++) {
            res.add(this.str.substring(i, j));
          }
       }

       return res;
    }

    //Checks if the string is having unique characters
    private boolean isUniqueChars(String substring) {
      Set<Character> chars = new HashSet<>();
      int l = substring.length();
      
      for (int i=0; i<l; i++) {
         if (chars.contains(substring.charAt(i))) {
            return false;
         }
         chars.add(substring.charAt(i));
      }
      return true;
    }

    public String getResult() {
      //Get all substrings
      Set<String> substrings = this.getAllUniqueSubstrings();

      //check their lengths, and return string with highest length
      String result = null;
      int maxLen = 0;
      for (String substring : substrings) {
         if (this.isUniqueChars(substring)) {
            if (substring.length() > maxLen) {
               result = substring;
               maxLen = substring.length();
            }
         }
      }

      return result;
    }
}

Complexity

Complexity on finding all substrings: O(n^2)
Complexity on iterating over all substrings: O(n)
Complexity on finding whether substring is having unique characters: O(n)

Overall complexity of this program is: O(n^2)

2. A Better Approach ~ O(n)

In above approach, our main time is in calculating substrings and checking if it has unique character. Lets have a look at the Sliding Window solution. We can maintain a HashSet to check if our substring has unique character or not. And, two variables for maintaining a sliding window.

Sliding Window

Approach

We have a HashSet to check unique characters in continuous substring
We have two variables (i-start, j-end) to keep sliding window range.
Iterate over string
If the character is not found in HashSet, it will be added. And, sliding window end(variable j) will be incremented.
In the meanwhile, keep track of maximum length unique string.
If we found the character in HashSet, means we encounter a duplicate character. Remove the character from HashSet which is occuring as sliding window start (variable i) position.
In a sliding window, we always have unique characters of a substring. And, it will be reduce to zero characters on encountering a duplicate character.

Lets look at the code:

public class OptimizedSolution {
    private String str;
    
    public OptimizedSolution(String str) {
       this.str = str;
    }

    public String getResult() {
      int l = this.str.length();
      Set<Character> set = new HashSet<>();
      int i=0;
      int j=0;
      String maxStr = "";

      while (i < l && j < l) {
         if (!set.contains(this.str.charAt(j))) {
            set.add(this.str.charAt(j));
            j++;
            String s = this.str.substring(i, j);
            if (s.length() > maxStr.length()) {
               maxStr = s;
            }
         }
         else {
            set.remove(this.str.charAt(i));
            i++;
         }
      }

      return maxStr;
    }
}

Complexity

Overall complexity of this program is: O(n) At the max, the program will run for O(2n) which is nearly equal to O(n)

3. Best Approach - O(n)

In above approach, we are moving left side of sliding window one by one on encountering a duplicate character. Our objective is to move that left pointer of sliding window to the occurance of duplicate character plus 1.

Sliding Window

Lets look at the code.

public class MoreOptimizedSolution {
    private String str;
    public MoreOptimizedSolution(String str) {
        this.str = str;
    }

    public String getResult() {
        Map<Character, Integer> map = new HashMap<>();
        int l = this.str.length();
        int i=0; int j=0;
        String maxStr = "";

        for (; j<l; j++) {
            if (map.containsKey(this.str.charAt(j))) {
                i = Math.max(map.get(this.str.charAt(j)), i);
            }

            if (j-i+1 > maxStr.length()) {
                maxStr = this.str.substring(i, j+1);
            }

            map.put(this.str.charAt(j), j+1);
        }

        return maxStr;
    }
}