LeetCode 159. Longest Substring with At Most Two Distinct Characters - Google - HashMap, Two Pointers, String

The main idea is to maintain a sliding window with 2 unique characters. 

The key is to store the last occurrence of each character as the value in the hashmap.

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

	class Solution {
	public int lengthOfLongestSubstringTwoDistinct(String s) {
	// filter abnormal inputs
	if (s == null) {
	return 0;
	}
	if (s.length() < 3) {
	return s.length();
	}
	// core logic
	int start = 0;
	int end = 0;
	int maxLength = Integer.MIN_VALUE;
	HashMap<Character, Integer> map = new HashMap<>();
	for(int i = 0; i < s.length(); i++) {
	Character key = s.charAt(i);
	if (!map.containsKey(key) && map.size() == 2) {
	// situation 3
	int leftMost = Integer.MAX_VALUE;
	for(int value: map.values()) {
	leftMost = Math.min(leftMost, value);
	}
	start = leftMost + 1;
	map.remove(s.charAt(leftMost));
	}
	map.put(key, end);
	maxLength = Math.max(maxLength, end - start + 1);
	end++;
	}
	return maxLength;
	}
	}

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