题目

Given a string, find the length of the longest substring without repeating characters.

Examples:

Given “abcabcbb”, the answer is “abc”, which the length is 3.

Given “bbbbb”, the answer is “b”, with the length of 1.

Given “pwwkew”, the answer is “wke”, with the length of 3. Note that the answer must be a substring, “pwke” is a subsequence and not a substring.

也就是说给定一个字符串，输出不包含重复字母的最长子串长度。

思路

遍历一次字符串，O(n)复杂度下可以解决。主要思路就是在遍历的过程中

1. 记录每个字母上一次出现的位置

2. 维持一个从当前位置往前数不包含重复字母的子串，记录这个字串的起止位置start, end

遍历的过程中就是根据相应位置字母是否出现过，以及上次出现的位置，不断更新start, end的过程。

代码

可以到github上查看: https://github.com/lcy362/Algorithms/tree/master/src/main/java/com/mallow/algorithm

<span class="hljs-keyword">import</span> java.util.HashMap;

<span class="hljs-javadoc">/**
 * leetcode 3
 * https://leetcode.com/problems/longest-substring-without-repeating-characters/
 * Created by lcy on 2017/2/15.
 */</span>
<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">LongestSubstringNotRepeat</span> {</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">int</span> <span class="hljs-title">lengthOfLongestSubstring</span>(String s) {
        <span class="hljs-keyword">if</span> (s.length() &lt;= <span class="hljs-number">1</span>) {
            <span class="hljs-keyword">return</span> s.length();
        }
        HashMap&lt;Character, Integer&gt; charPos = <span class="hljs-keyword">new</span> HashMap&lt;&gt;();
        <span class="hljs-keyword">char</span>[] chars = s.toCharArray();
        <span class="hljs-keyword">int</span> len = <span class="hljs-number">0</span>;
        <span class="hljs-keyword">int</span> max = <span class="hljs-number">0</span>;
        <span class="hljs-keyword">int</span> start = <span class="hljs-number">0</span>;
        <span class="hljs-keyword">int</span> end = <span class="hljs-number">0</span>;
        <span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i &lt; chars.length; i++) {
            <span class="hljs-keyword">if</span> (charPos.containsKey(chars[i])) {
                <span class="hljs-keyword">int</span> tempstart = charPos.get(chars[i]) + <span class="hljs-number">1</span>;
                <span class="hljs-keyword">if</span> (tempstart &gt; start) {
                    start = tempstart;
                }
                end++;
                len = end - start;
            } <span class="hljs-keyword">else</span> {
                len++;
                end++;
            }
            charPos.put(chars[i], i);
            <span class="hljs-keyword">if</span> (len &gt; max) {
                max = len;
            }
        }
        <span class="hljs-keyword">return</span> max;
    }

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span>(String args[]) {
        LongestSubstringNotRepeat l = <span class="hljs-keyword">new</span> LongestSubstringNotRepeat();
        System.out.println(l.lengthOfLongestSubstring(<span class="hljs-string">"abcabcbb"</span>));
        System.out.println(l.lengthOfLongestSubstring(<span class="hljs-string">"bbbbb"</span>));
        System.out.println(l.lengthOfLongestSubstring(<span class="hljs-string">"pwwkew"</span>));
        System.out.println(l.lengthOfLongestSubstring(<span class="hljs-string">"abba"</span>));
    }