Description

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

Example 1:

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

Example 2:
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Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:
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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.

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Constraints:

0 <= s.length <= 5 * 104
s consists of English letters, digits, symbols and spaces.

Approach: Sliding Window

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class Solution {
    public int lengthOfLongestSubstring(String s) {
        int n = s.length();
        Set<Character> set = new HashSet<>();
        int ans = 0, i = 0, j = 0;
        while (i < n && j < n) {
            if (!set.contains(s.charAt(j))) {
                set.add(s.charAt(j++));
                ans = Math.max(ans, j - i);
            } else {
                set.remove(s.charAt(i++));
            }
        }
        return ans;
    }
}
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class Solution {
    public int lengthOfLongestSubstring(String s) {
        int start = 0;
        int[] lastIndex = new int[128]; // assuming ASCII characters
        for(int i=0; i<128; i++){
            lastIndex[i] = -1;  // initialize all characters' last index as -1
        }
        int max = 0;
        for(int end=0; end<s.length(); end++){
            char c = s.charAt(end);
            if(lastIndex[c] >= start) start = lastIndex[c] + 1;
            lastIndex[c] = end;
            if(end - start + 1 > max) max = end - start + 1;
        }
        return max;
    }
}