340. Longest Substring with At Most K Distinct Characters

Description

Given a string s and an integer k, return the length of the longest substring of s that contains at most k distinct characters.

 

Example 1:

Input: s = "eceba", k = 2
Output: 3
Explanation: The substring is "ece" with length 3.

Example 2:

Input: s = "aa", k = 1
Output: 2
Explanation: The substring is "aa" with length 2.

 

Constraints:

  • 1 <= s.length <= 5 * 104
  • 0 <= k <= 50

Solutions

Solution 1

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

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

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

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