Find the length of the longest substring without repeating character

find_substring.py

# program to find lenght of longest substring without repeating characters.  
def findLongestSubstring(string): 
  
    l = len(string)  
    st = 0 # starting point of current substring.  
      
    maxlen = 0 # maximum length of substring   
    start = 0  # starting index of maximum  
   
    d = dict()  # dictionary to store last occurrence
    
    d[string[0]] = 0 # store occurrence of first character
  
    for i in range(1, l):  
        if string[i] not in d:  # if character not in dict then add it
            d[string[i]] = i  
  
        else: 
            if d[string[i]] >= st:    
                currlen = i - st  
                if maxlen < currlen:  
                    maxlen = currlen  
                    start = st  
                st = d[string[i]] + 1
              
            d[string[i]] = i  
          
    if maxlen < i - st:  
        maxlen = i - st  
        start = st  
    print(string[start : start + maxlen])
    return len(string[start : start + maxlen]) # insted of returning length 
    # you can return string    

  
string = "ABACDBFB"
print(findLongestSubstring(string))  
 # the output will be "ACDBF"