USACO-T023: Sorting a Three-valued Sequence

/*
ID: mickyta1
TASK: sort3
LANG: C++
*/

#include <bits/stdc++.h>
using namespace std;

const int N = 1000 + 5;

deque<int> ones;
vector<int> lft, sortedLft;
int arr[N], sorted[N];

int main(){
    freopen("sort3.in", "r", stdin);
    freopen("sort3.out", "w", stdout);

    int n;
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i){
        scanf("%d", &arr[i]);
        sorted[i] = arr[i];
    }
    sort(sorted + 1, sorted + n + 1);
    for(int i = 1; i <= n; ++i){
        if(arr[i] != sorted[i]){
            lft.push_back(arr[i]);
            sortedLft.push_back(arr[i]);
        }
    }
    sort(sortedLft.begin(), sortedLft.end());
    int cnt = 0;
    for(int i = 0; i < lft.size(); ++i){
        if(lft[i] == 1){
            ones.push_back(i);
        }
    }
    int i;
    for(i = 0; i < lft.size() && sortedLft[i] == 1; ++i){
        if(lft[i] == 2){
            swap(lft[i], lft[ones.front()]);
            ones.pop_front();
        } else if(lft[i] == 3){
            swap(lft[i], lft[ones.back()]);
            ones.pop_back();
        }
        ++cnt;
    }
    for(; i < lft.size() && sortedLft[i] == 2; ++i){
        if(lft[i] == 3){
            ++cnt;
        }
    }
    cout << cnt << '\n';

    fclose(stdin);
    fclose(stdout);
    return 0;
}