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

Question

#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 i = 0; i < number.size(); i++ )
{
for( int j = i + 1; j < number.size() + 1; j++ )
{
if( number[ i ] != 0 && number[ i ] == number[ j ] )
{
repeated[ i ]++;
unique_number[ i ] = number[ i ];
}
else
unique_number[ i ] = number[ i ];
if( j == number.size() )
{
for( int z = 0; z < number.size(); z++ )
{
if( number[ z ] == unique_number[ i ] )
number[ z ] = 0;
}
}
}
}
for( int i = 0; i < number.size(); i++ )
{
if( ( unique_number[ i ] != 0 && repeated[ i ] == 1 ) || ( unique_number[ i ] != 0 && repeated[ i ] % 2 != 0 ) )
{
numbersAreCorrect = false;
cout << unique_number[ i ] << endl;
break;
}
else if( repeated[ i ] == 1 )
numbersAreCorrect = true;
else if( repeated[ i ] % 2 != 0 )
{
numbersAreCorrect = false;
cout << repeated[ i ] << endl;
break;
}
else if( repeated[ i ] % 2 == 0 )
numbersAreCorrect = true;
}
if( numbersAreCorrect == true )
cout << "0" << endl;
return 0;
}
This program gets positive integers from user, checks if an integer is repeated 2k( even) times or 2k+1(odd) times. if latter is true, it prints the integer , else it prints 0; I used 20000 inputs and it takes more than 10 seconds to evaluate.. I need to know how to make it process faster.
for example this input results in "0" : 1 2 2 1
and this results in "3" : 1 2 2 1 3

Answer 1

How about you first sort the thing.
Then you only need to do a single for loop instead of two because to find all the repetitions you just count consecutive occurrences.
Failing that use a set or map. Again you'll drop to O(NlogN) instead of O(N^2).