Problem p03137 (C++)

Buggy submission (status: Time Limit Exceeded):
// include
//------------------------------------------
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <stack>
#include <stdlib.h>
#include <string>
#include <utility>
#include <vector>
using namespace std;
// conversion
//------------------------------------------
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
// math
//-------------------------------------------
template <class T> inline T sqr(T x) { return x * x; }
// typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> P;
typedef long long ll;
// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
// clear memory
#define CLR(a) memset((a), 0, sizeof(a))
// debug
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
// chmax chmin
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

int main(void) {
  int N, M;
  cin >> N >> M;
  if (N >= M) {
    cout << 0 << endl;
    return 0;
  }
  vector<int> X(M);
  REP(i, M) cin >> X[i];
  sort(X.begin(), X.end());
  vector<int> step;
  FOR(i, 1, M) step.push_back(X[i] - X[i - 1]);
  REP(i, N - 1) {
    sort(step.begin(), step.end());
    reverse(step.begin(), step.end());
    step[0] = 0;
  }

  sort(step.begin(), step.end());
  reverse(step.begin(), step.end());
  int ans = 0;
  REP(i, M) {
    int elem = step[i];
    if (elem == 0)
      break;
    ans += elem;
  }

  cout << ans << endl;

  return 0;
}

Fixed submission:
// include
//------------------------------------------
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <stack>
#include <stdlib.h>
#include <string>
#include <utility>
#include <vector>
using namespace std;
// conversion
//------------------------------------------
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
// math
//-------------------------------------------
template <class T> inline T sqr(T x) { return x * x; }
// typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> P;
typedef long long ll;
// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
// clear memory
#define CLR(a) memset((a), 0, sizeof(a))
// debug
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
// chmax chmin
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

int main(void) {
  int N, M;
  cin >> N >> M;
  if (N >= M) {
    cout << 0 << endl;
    return 0;
  }
  vector<int> X(M);
  REP(i, M) cin >> X[i];
  sort(X.begin(), X.end());
  vector<int> step;
  FOR(i, 1, M) step.push_back(X[i] - X[i - 1]);
  sort(step.begin(), step.end());
  reverse(step.begin(), step.end());
  REP(i, N - 1) step[i] = 0;

  sort(step.begin(), step.end());
  reverse(step.begin(), step.end());
  int ans = 0;
  REP(i, M) {
    int elem = step[i];
    if (elem == 0)
      break;
    ans += elem;
  }

  cout << ans << endl;

  return 0;
}