Use stable_partition algorithms: preserving relative order in each group : stable_partition « STL Algorithms Modifying sequence operations « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Modifying sequence operations
3. stable_partition

#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
#include <iterator>
using namespace std;

bool above40(int n) { return (n > 4); }

int main()
{
  const int N = 7;
  int array0[N] = {5, 3, 1, 7, 6, 4, 2};
  int array1[N];

  copy(&array0[0], &array0[N], &array1[0]);

  ostream_iterator<int> out(cout, " ");

  cout << "Original sequence:                 ";
  copy(&array1[0], &array1[N], out); cout << endl;

  // Partition array1, putting numbers greater than 4
  // first, followed by those less than or equal to 4,
  // preserving relative order in each group:
  int* split = stable_partition(&array1[0], &array1[N], above40);

  cout << "Result of stable partitioning:     ";
  copy(&array1[0], split, out); cout << "| ";
  copy(split, &array1[N], out); cout << endl;
  return 0;
}

Original sequence:                 5 3 1 7 6 4 2
Result of stable partitioning:     5 7 6 | 3 1 4 2