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159. Longest Substring with At Most Two Distinct Characters πŸ”’

Description

Given a string s, return the length of the longest substring that contains at most two distinct characters.

 

Example 1:

Input: s = "eceba"
Output: 3
Explanation: The substring is "ece" which its length is 3.

Example 2:

Input: s = "ccaabbb"
Output: 5
Explanation: The substring is "aabbb" which its length is 5.

 

Constraints:

  • 1 <= s.length <= 105
  • s consists of English letters.

Solutions

Solution 1

Thinking

Longest substring with at most two distinct characters. \(n\le 10^5\), so enumerating endpoints is \(O(n^2)\). The distinct count grows with the right end and shrinks with the left. Extend right while counting; when there are more than two kinds, move left. Update the answer on a valid window.

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class Solution:
    def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
        cnt = Counter()
        ans = j = 0
        for i, c in enumerate(s):
            cnt[c] += 1
            while len(cnt) > 2:
                cnt[s[j]] -= 1
                if cnt[s[j]] == 0:
                    cnt.pop(s[j])
                j += 1
            ans = max(ans, i - j + 1)
        return ans

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class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        Map<Character, Integer> cnt = new HashMap<>();
        int n = s.length();
        int ans = 0;
        for (int i = 0, j = 0; i < n; ++i) {
            char c = s.charAt(i);
            cnt.put(c, cnt.getOrDefault(c, 0) + 1);
            while (cnt.size() > 2) {
                char t = s.charAt(j++);
                cnt.put(t, cnt.get(t) - 1);
                if (cnt.get(t) == 0) {
                    cnt.remove(t);
                }
            }
            ans = Math.max(ans, i - j + 1);
        }
        return ans;
    }
}

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class Solution {
public:
    int lengthOfLongestSubstringTwoDistinct(string s) {
        unordered_map<char, int> cnt;
        int n = s.size();
        int ans = 0;
        for (int i = 0, j = 0; i < n; ++i) {
            cnt[s[i]]++;
            while (cnt.size() > 2) {
                cnt[s[j]]--;
                if (cnt[s[j]] == 0) {
                    cnt.erase(s[j]);
                }
                ++j;
            }
            ans = max(ans, i - j + 1);
        }
        return ans;
    }
};

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func lengthOfLongestSubstringTwoDistinct(s string) (ans int) {
    cnt := map[byte]int{}
    j := 0
    for i := range s {
        cnt[s[i]]++
        for len(cnt) > 2 {
            cnt[s[j]]--
            if cnt[s[j]] == 0 {
                delete(cnt, s[j])
            }
            j++
        }
        ans = max(ans, i-j+1)
    }
    return
}

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