Longest Substring Without Repeating Characters

Description

Given a string s, find the length of the longest

substring

without repeating characters.

Example 1:

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

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.

Constraints:

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

Code

Time Complexity: $O(n)$, Space Complexity: $O(n)$

和 Contiguous Array 中一樣將起始點設為 -1（i = -1），因為考慮到若整個 array 都是答案的情況。

if(mp[s[j]] > i) 是為了 abba 這種 case，這時遇到後面的 a 時就不應該將 i 移往前面的那個 a。

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        unordered_map<char, int> mp;
        int i = -1, j = 0;
        int res = 0;
        for(int j = 0; j < s.length(); j++) {
            if(mp.find(s[j]) != mp.end()) {
                if(mp[s[j]] > i)
                    i = mp[s[j]];
            } 
            mp[s[j]] = j;
            res = max(res, j - i);
        }
        return res;
    }
};

Source

• Longest Substring Without Repeating Characters - LeetCode