3. Longest Substring Without Repeating Characters

Posted on 2 mins read

1. Description

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

2. Example

Example 1

Input: s = “abcabcbb”
Output: 3
Explanation: The answer is “abc”, with the length of 3. Note that “bca” and “cab” are also correct answers.

Example 2

Input: s = “bbbbb”
Output: 1
Explanation: The answer is “b”, with the length of 1.

Example 3

Input: s = “pwwkew”
Output: 3
Explanation: The answer is “wke”, with the length of 3.
Notice that the answer must be a substring, “pwke” is a subsequence and not a substring.

3. Constraints

  • 0 <= s.length <= 5 * 10$^4$
  • s consists of English letters, digits, symbols and spaces.

4. Solutions

Sliding Window

n = str.size()
Time complexity: O(n)
Space complexity: O(1)

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class Solution {
public:
    int lengthOfLongestSubstring(const string &str) {
        const int n = str.size();
        int max_length = 0;
        array<int, 128> count{};
        for (int left = 0, right = 0; right < n; ++right) {
            ++count[str[right]];

            while (count[str[right]] > 1) {
                --count[str[left]];
                ++left;
            }

            max_length = max(max_length, right - left + 1);
        }

        return max_length;
    }
};
Optimized Sliding Window

n = str.size()
Time complexity: O(n)
Space complexity: O(1)

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class Solution {
public:
    int lengthOfLongestSubstring(const string &str) {
        const int n = str.size();
        array<int, 128> prev_index;
        prev_index.fill(-1);

        int max_length = 0;
        for (int left = 0, right = 0; right < n; ++right) {
            left = max(left, prev_index[str[right]] + 1);
            max_length = max(max_length, right - left + 1);
            prev_index[str[right]] = right;
        }

        return max_length;
    }
};
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