题目

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.
Example 4:

Input: s = “”
Output: 0

Constraints:

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

解法：双指针

双指针i，j
用哈希表维护【i，j】区间内字符出现次数
对于下一个字符：

• 如果出现过，那么i右移
• 如果没有出现过，那么j右移

时间复杂度O(n)，空间复杂度O(1)

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int ans = 0;
        int f[256] = {0};
        int l = 0, r = -1;
        while (r + 1 < s.size()) {
            if (f[s[r + 1]]) {
                f[s[l++]]--;
            }
            else {
                f[s[++r]]++;
                ans = max(ans, r - l + 1);
            }
        }
        return ans;
    }
};