【LeetCode】159. Longest Substring with At Most Two Distinct Characters

時間 2019-11-07

標籤 LeetCode longest substring distinct characters 简体版

原文原文鏈接

Difficulty: Hard

Description

Given a string S, find the length of the longest substring T that contains at most two distinct characters.
For example,
Given S = 「eceba」,
T is "ece" which its length is 3.java

Intuition

方法一：假設有一個滑動窗口，窗口左右兩邊的下標分別記爲left和right。遍歷字符串，依次肯定left和right。具體實現見代碼（稍微難理解一點）post

方法二：仍是引入一個滑動窗口，窗口左右兩邊的下標分別記爲left和right。再引入一個numDistinct變量，用於記錄窗口中已經出現的不一樣字符次數。此外，使用一個int[258]來模擬哈希表，記錄每一個字符出現的次數count。ui

　　遍歷字符串（right右移），當遇到的字符出現次數爲0時，numDistinct++; 當numDistinct的次數超過2時，說明left須要往右移，在left往右移的過程當中，減小所指字符的count，最終使得numDistinct=2，才暫停left的右移。指針

　　方法二容易理解，且能夠用於求解k個不一樣字符組成的最長子字符。下面代碼中方法二就是實現k個不一樣字符的狀況。htm

Solution

	//Method1: Longest Substring with At Most Two Distinct Characters 
	public static int lengthOfLongestSubstringTwoDistinct1(String s) {
		if(s==null || s.length()<=0)
			return 0;
		int left=0;
		int right=-1;
		int maxLen=0;
		for(int i=1;i<s.length();i++) {
			if(s.charAt(i)==s.charAt(i-1))	continue;
			if(right!=-1 && s.charAt(i)!=s.charAt(right)) {
				maxLen=Math.max(maxLen, i-left);
				left=right+1;
			}
			right=i-1;
		}
		return Math.max(s.length()-left,maxLen);
	}
	
	//Method2: Longest Substring with At Most k Distinct Characters 
	public static int lengthOfLongestSubstringTwoDistinct2(String s,int k) {
		if(s==null || s.length()<=0)
			return 0;
		int[] count=new int[256];
		int numDistinct=0;
		int maxLen=0;
		int left=0;
		for(int right=0;right<s.length();right++) {
			if(count[s.charAt(right)]==0) numDistinct++;
			count[s.charAt(right)]++;
			while(numDistinct>k) {
				count[s.charAt(left)]--;
				if(count[s.charAt(left)]==0) numDistinct--;
				left++;
			}
			maxLen=Math.max(maxLen, right-left+1);
		}
		return maxLen;
	}