USACO 2.1 Sorting A Three-Valued Sequence (sort3)

//Main Idear
//The problem is quite easy, the key point is to consider all kinds of situation.
//We first calculate the number of 1,2,3,namely, counter1,counter2,counter3
//So, when the sequence is in non-decreasing order, number 1 shoud be in place(start from 0)
//[0,counter1-1],2 in [counter1,counter1+counter2-1] and 3 in [counter1+counter2,sequence_lenght-1]
//Then we can count then number in wrong place and use the min swap step to put them in order.

/*
ID: haolink1
PROG: sort3
LANG: C++
*/

//#include <iostream>
#include <fstream>

using namespace std;

int main(){
ifstream fin("sort3.in");
short len = 0;
fin >> len;
short* numbers = new short[len];
for(short i = 0; i < len; i++){
fin >> numbers[i];
}
//calculate the number of 1,2,3 and store them in counter[0],counter[1],counter[2] respectively
short counter[3] = {0};
for(short i = 0; i < len; i++){
counter[numbers[i]-1]++;
}
//    for(short i = 0; i < 3; i++){
//        cout<<counter[i]<<endl;
//    }
//caltulate the number in the wrong place.
//Eg: number 2, and 3 shoudn't appear in the palce that's lower then counter[0]
short num_2_in_1 = 0;
short num_3_in_1 = 0;
for(short i = 0; i< counter[0]; i++){
if(numbers[i] == 2)
num_2_in_1++;
if(numbers[i] == 3)
num_3_in_1++;
}
short num_1_in_2 = 0;
short num_3_in_2 = 0;
for(short i = counter[0]; i < counter[0]+counter[1]; i++){
if(numbers[i] == 1)
num_1_in_2++;
if(numbers[i] == 3)
num_3_in_2++;
}
short num_1_in_3 = 0;
for(short i =counter[0]+counter[1]; i < len; i++){
if(numbers[i] == 1)
num_1_in_3++;
}
//    cout<<num_2_in_1<<" "<<num_3_in_1<<" "<<num_1_in_2<<" "<<num_3_in_2<<" "<<num_1_in_3<<endl;
//calculate the swap times we need to make the sequence in order.
short swap_num = 0;
if(num_2_in_1 > 0){
if(num_2_in_1 <=  num_1_in_2){
swap_num += num_2_in_1;
num_1_in_2 -= num_2_in_1;
num_2_in_1 = 0;
}else{
swap_num += num_1_in_2;
num_2_in_1 -= num_1_in_2;
num_1_in_2 = 0;
}
if(num_2_in_1 > 0){
swap_num += 2*num_2_in_1;
num_1_in_3 -= num_2_in_1;
num_3_in_2 -= num_2_in_1;
num_2_in_1 = 0;
}
}
if(num_3_in_1 > 0){
if(num_3_in_1 <= num_1_in_3){
swap_num += num_3_in_1;
num_1_in_3 -= num_3_in_1;
num_3_in_1 = 0;
}else{
swap_num += num_1_in_3;
num_3_in_1 -= num_1_in_3;
num_1_in_3 = 0;
}
if(num_3_in_1 > 0){
swap_num += 2*num_3_in_1;
num_1_in_2 -= num_3_in_1;
//num_2_in_3 -= num_3_in_1;
num_3_in_1 = 0;
}
}
if(num_3_in_2 > 0){
swap_num += num_3_in_2;
}
ofstream fout("sort3.out");
fout<<swap_num<<endl;
delete[] numbers;
return 0;
}