55-The first non-repeating character in a character stream

When you read only the first two characters "go" from the character stream, the first character that appears only once is ' G '. When the first six characters "Google" are read out of the stream, a character that appears only 1 times is "L".

The first is to record the number of occurrences of a character , in order to achieve O (1) lookup, using simple hash table storage. Use occurences[256] to record the number of occurrences of a character. Set up:
Occurences[i] = 0, the character does not appear;
Occurences[i] = 1, the character appears once;
Occurences[i] = 2, the character appears 2 times or greater than 2 times
The character that appears once, using the FIFO queue record. In the subsequent character input process, you may enter a character that already exists once, there may be more than one occurrence of the character in the queue, so when you take the top element of the queue, you should check again if the element appears 1 times, if not, the queue pops until it finds a character index that appears only once.

time Complexity O (1),
Spatial complexity: occurences[256] space is constant; the queue will only have a maximum of 256 characters (only the first occurrence of the character). So space complexity O (1)

#include <iostream>#include <queue>using namespace STD;classCharstatics {Private:unsigned intoccurences[ the];intIndex Queue<int>Index_queue; Public: Charstatics () {index =-1; for(inti =0; I <=255; i++) Occurences[i] =0; }voidInsertchar (CharCH) {if(Occurences[ch] = =0) {Occurences[ch] =1;//First occurrence, set number of occurrences, press in queueIndex_queue.push (CH); }Else{Occurences[ch] =2;//2nd or more occurrences}     }CharFirstapperingonce () {//Find the character that appears first only once and use index to point to         while(!index_queue.empty () && Occurences[index_queue.front ()]! =1) {Index_queue.pop (); }if(!index_queue.empty ()) index = Index_queue.front ();Elseindex =-1;//No characters that appear only once        if(Index = =-1)return ' + ';returnindex+' + '; }};intMain () {charstatics str; Str. Insertchar (' G ');cout<< Str.    Firstapperingonce () << Endl; Str. Insertchar (' O ');cout<< Str.    Firstapperingonce () << Endl; Str. Insertchar (' O ');cout<< Str.    Firstapperingonce () << Endl; Str. Insertchar (' G ');cout<< Str.    Firstapperingonce () << Endl; Str. Insertchar (' l ');cout<< Str.    Firstapperingonce () << Endl; Str. Insertchar (' E ');cout<< Str. Firstapperingonce () << Endl;}

Output:

gggNULll

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

55-The first non-repeating character in a character stream