LeetCode 3. Longest Substring Without Repeating Characters

Problem:

This system give you a long string,you should work out the longest substring without
repeating characters.

Such as :

Given "pwwkew", the answer is "wke", with the length of 3.

The first solution:

class Solution ( object ):
    def Check ( self , tmp ):
        char_set = set ( tmp )
        if len ( tmp ) != char_set.__len__ ():
            return {'answer': False , 'result': ''}
        else:
            return {'answer': True , 'result': char_set}
    def lengthOfLongestSubstring ( self , s ):
        length = len ( s )
        flag = 0
        for each in range ( length ):
            for ebch in range ( each + flag , length ):
                if ebch - each >= flag:
                    # print "%d\t%d" %(each,ebch)
                    # print s[each:ebch]
                    check = self.Check ( s[ each:ebch] )
                    # print check
                    if check[ 'answer' ] == True:
                        flag = ebch  - each
                        for t in s[ ebch: ]:
                            if t not in check[ 'result' ]:
                                check[ 'result' ].add ( t )
                                flag += 1
                            else:
                                ebch = length - 1
                                break
                    else:
                        break
        return flag

As what you have seen above,the first solution uses three layers of circulations.Thus,the sum of the consumption is O(n³). As for a long string, a long time is needed to work it out.

The second solution

class Solution ( object ):
    def lengthOfLongestSubstring ( self , s ):
        length = len ( s )
        i = 0
        j = 0
        answer = 0
        char_set = set ()
        while i < length and j < length:
            if s[ j ]  not in char_set:
                char_set.add(s[j])
                j += 1
                answer = max(answer,j - i)
            else:
                char_set.remove(s[i])
                i += 1
        return answer

As for the second methods,it uses a sliding windows to get the substring.In order to check whether the substring has repeating characters, set is a very essential tool according to its feature.
The sliding windows is the boundary of [i,j) (left-closed and right-opened ).When we check whether the substring s[i,j+1] has repeating characters, the result of substring s[i,j) which we have worked out is helpful. we only need to test whether s[j] is in the old set.