#include <string>
#include <vector>
#include <map>
#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

class BearPasswordLexic {
  public:
    string findPassword(vector <int> x);
};

string BearPasswordLexic::findPassword(vector <int> x)
{
  int i;
  int ws;                         /* Keeping track of the password size. */
  int j;                          /* This is the largest index of x such that x[j] > 0 */
  char c;                         /* We'll set this to 'a' or 'b' when building the password. */
  string rv;                      /* The password (return value) */
  string k;                       /* This helps us build the password */
  map <int, int> S;               /* The map whose key is contiguous letters, and
                                     whose val is the number of substrings with thos letters. */
  map <int, int>::iterator ait;   /* Iterator for a's. */
  map <int, int>::iterator bit;   /* Iterator for b's. */

  if (x[0] != (int) x.size()) return "";

  /* We are going to build S. */

  ws = 0;
  while (ws < x.size()) {

    /* Find j */

    for (j = x.size()-1; j >= 0 && x[j] == 0; j--) ;
    if (j < 0) return "";
    ws += (j+1);
 
    /* Add j's string to the map */

    S[j+1]++;

    /* Now subtract out the j substring for all indices less than or equal to j */

    i = 1;
    while (j >= 0) {
      if (x[j] < i) return "";
      x[j] -= i;
      j--;
      i++;
    }
  }

  /* Double-check: If any elements of x are non-zero, then x was illegal */

  for (i = 0; i < x.size(); i++) if (x[i] != 0) return "";

  /* Build the return string. I'm setting ait to be an iterator to the high end of S,
     and bit to be an iterator to the low end. */

  rv = "";
  c = 'a';
  ait = S.end();
  ait--;
  bit = S.begin();

  /* Build the password from S.  The a's come from the high end, 
     and the b's come from the low end. */

  while (rv.size() < ws) {
    if (c == 'a') {
      k.clear();
      k.resize(ait->first, 'a');
      rv += k;
      ait->second--;
      if (ait->second == 0) ait--;
      c = 'b';
    } else {
      k.clear();
      k.resize(bit->first, 'b');
      rv += k;
      bit->second--;
      if (bit->second == 0) bit++;
      c = 'a';
    }
  }
  
  return rv;
}