Frequency

159. Longest Substring with At Most Two Distinct Characters

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.

int lengthOfLongestSubstringTwoDistinct(String s) {
    int n = s.length();
    int ans = 0;
    Map<Character, Integer> freq = new HashMap<>();
    for (int i = 0, j = 0; j < n; j++) {
        freq.merge(s.charAt(j), 1, Integer::sum);
        while (freq.size() > 2) {
            char c = s.charAt(i++);
            if (freq.merge(c, -1, Integer::sum) == 0) {
                freq.remove(c);
            }
        }
        ans = Math.max(ans, j - i + 1);
    }
    return ans;
}

340. Longest Substring with At Most K Distinct Characters

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

int lengthOfLongestSubstringKDistinct(String s, int k) {
    int n = s.length();
    Map<Character, Integer> freq = new HashMap<>();
    int ans = 0;
    for (int i = 0, j = 0; j < n; j++) {
        freq.merge(s.charAt(j), 1, Integer::sum);
        while (freq.size() > k) {
            char c = s.charAt(i);
            if (freq.merge(c, -1, Integer::sum) == 0) {
                freq.remove(c);
            }
            i++;
        }
        ans = Math.max(ans, j - i + 1);
    }
    return ans;
}

904. Fruit Into Baskets

You are visiting a farm that has a single row of fruit trees arranged from left to right. The trees are represented by an integer array fruits where fruits[i] is the type of fruit the ith tree produces.

You want to collect as much fruit as possible. However, the owner has some strict rules that you must follow:

• You only have two baskets, and each basket can only hold a single type of fruit. There is no limit on the amount of fruit each basket can hold.
• Starting from any tree of your choice, you must pick exactly one fruit from every tree (including the start tree) while moving to the right. The picked fruits must fit in one of your baskets.
• Once you reach a tree with fruit that cannot fit in your baskets, you must stop.

Given the integer array fruits, return the maximum number of fruits you can pick.

public int totalFruit(int[] fruits) {
    // Hash map 'basket' to store the types of fruits.
    Map<Integer, Integer> basket = new HashMap<>();
    int ans = 0;
    // Add fruit from j side (j) of the window.
    for (int i = 0, j = 0; j < fruits.length; ++j) {
        basket.put(fruits[j], basket.getOrDefault(fruits[j], 0) + 1);

        while (basket.size() > 2) {
            basket.put(fruits[i], basket.get(fruits[i]) - 1);
            if (basket.get(fruits[i]) == 0)
                basket.remove(fruits[i]);
            i++;
        }
        ans = Math.max(j - i + 1, ans);
    }

    return ans;
}