leetcode-Longest Substring Without Repeating Characters

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

import java.util.HashMap;import java.lang.Math;class Solution {    public int lengthOfLongestSubstring(String s) {        HashMap<Character,Integer> map = new HashMap<Character,Integer>();        if(s==null || s.length()<=0){            return 0;        }        int j=0;        int max = 0;        for(int i=0; i<s.length(); i++){           if(map.containsKey(s.charAt(i))){               j = Math.max(j,map.get(s.charAt(i))+1);           }            map.put(s.charAt(i),i);            max = Math.max(max,i-j+1);        }        return max;    }}

**这道题的思路也是利用hashmap，相当于有两个指针，第二个指针一直移动，当移动到hashmap中有目前的这个字符时，移动第一个指针，在这种移动的过程中，将max值保存下来，这种方法的时间复杂度时O(n)
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