3. Longest Substring Without Repeating Characters

自己方法复杂度比较高

可以O(n)的方法，遍历一遍 用两个指针记录maxstring的位置 然后遇到跟hashmap里重复的就更新指针
https://leetcode.com/problems/longest-substring-without-repeating-characters/discuss/1729/11-line-simple-Java-solution-O(n)-with-explanation

 

 

 

//Old
class Solution {
    public int lengthOfLongestSubstring(String s) {
        char[] arr = s.toCharArray();
        int max = 0;
        for(int i = 0; i < arr.length; i++) {
            max = Math.max(max, dfs(arr, new ArrayList<>(), i));
        }
        return max;
        
    }
    
    public int dfs(char[] arr, List<Integer> list, int i) {
        if(i >= arr.length || list.contains((int)arr[i])) return 0;
        list.add((int)arr[i]);
        return dfs(arr, list, i+1)+1;    
    }
    
}




//New
   public int lengthOfLongestSubstring(String s) {
        if (s.length()==0) return 0;
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        int max=0;
        for (int i=0, j=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;
    }