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# 159.Longest-Substring-with-At-Most-Two-Distinct-Characters

## 159. Longest Substring with At Most Two Distinct Characters

## 题目地址

<https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/>

## 题目描述

```
Given a string s , find the length of the longest substring t  that contains at most 2 distinct characters.

Example 1:

Input: "eceba"
Output: 3
Explanation: t is "ece" which its length is 3.
Example 2:

Input: "ccaabbb"
Output: 5
Explanation: t is "aabbb" which its length is 5.
```

## 代码

### Approach #1 Sliding Window + HashMap

```java
class Solution {
  public int lengthOfLongestSubstringTwoDistinct(String s) {
        int n = s.length;
    if (n < 3)    return n;

    int left = 0;
    int right = 0;

    HashMap<Character, Integer> hashmap = new HashMap<Character, Integer>();

    int max_len = 2;

    while (right < n) {
      if (hashmap.size() < 3) {
        hashmap.put(s.charAt(right), right++);
      }

      if (hashmap.size() == 3) {
        int del_idx = Collections.min(hashmap.values());
        hashmap.remove(s.charAt(del_idx));
        left = del_idx + 1;
      }

      max_len = Math.max(max_len, right - left);
    }

    return max_len;
  }
}
```


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