performance - What's making this program slow to process? (C++) -

#include <iostream> #include <vector> using namespace std;  int main() {     vector< int > number;     bool numbersarecorrect = false;      int input;      while( cin >> input )         number.push_back( input );      vector< int > unique_number( number.size(), 0 );     vector< int > repeated( number.size(), 1 );      for( int = 0; < number.size(); i++ )     {         for( int j = + 1; j < number.size() + 1; j++ )         {             if( number[ ] != 0 && number[ ] == number[ j ] )             {                 repeated[ ]++;                 unique_number[ ] = number[ ];             }             else                 unique_number[ ] = number[ ];              if( j == number.size() )             {                 for( int z = 0; z < number.size(); z++ )                 {                        if( number[ z ] == unique_number[ ] )                         number[ z ] = 0;                 }             }         }      }       for( int = 0; < number.size(); i++ )     {         if( ( unique_number[ ] != 0 && repeated[ ] == 1 ) || ( unique_number[ ] != 0 && repeated[ ] % 2 != 0 ) )         {                numbersarecorrect = false;             cout << unique_number[ ] << endl;             break;         }          else if( repeated[ ] == 1 )             numbersarecorrect = true;          else if( repeated[ ] % 2 != 0 )         {             numbersarecorrect = false;             cout << repeated[ ] << endl;             break;         }         else if( repeated[ ] % 2 == 0 )             numbersarecorrect = true;        }      if( numbersarecorrect == true )         cout << "0" << endl;      return 0; } 

this program gets positive integers user, checks if integer repeated 2k( even) times or 2k+1(odd) times. if latter true, prints integer , else prints 0; used 20000 inputs , takes more 10 seconds evaluate.. need know how make process faster. example input results in "0" : 1 2 2 1 , results in "3" : 1 2 2 1 3

how first sort thing. need single loop instead of 2 because find repetitions count consecutive occurrences.

failing use set or map. again you'll drop o(nlogn) instead of o(n^2).