cout set_precision(5) + 固定和 printf( "%lf.5" ，var) 之间的区别

我试图解决Area问题http://www.infoarena.ro/problema/aria，但对于第一个代码，我收到了"超过时间限制"，而对于第二个代码，一切都很好。那么，这两者之间有什么区别呢？

#include <fstream>
#include <vector>
#include <stdlib.h>
#include <iomanip>
#include <cmath>
#define max 1000000
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int N;
vector< pair<double,double> > points;
double tri_area(pair<double, double> p1, pair<double,double> p2){
    return p1.first * p2.second - p2.first * p1.second;
}
double find_area(){
    double area = 0;
    for(int i = 0; i < N-1; i++){
        area += tri_area(points[i],points[i+1]);
    }
    area += tri_area(points[N-1], points[0]);
    return abs(area)/2;
}
int main(){
    fin >> N;
    double x, y;
    for(int i = 0; i < N; i++){
        fin >> x >> y;
        points.push_back(make_pair(x,y));
    }
    fout << std::setprecision(5) << std::fixed << find_area() << "n";
    return 0;
}

第二：

#include <iostream>
#include <vector>
#include <stdlib.h>
#include <iomanip> 
#include <cmath>
#include <cstdio>
#define max 1000000
using namespace std;
int N;
vector< pair<double,double> > points;
double tri_area(pair<double, double> p1, pair<double,double> p2){
    return p1.first * p2.second - p2.first * p1.second;
}
double find_area(){
    double area = 0;
    for(int i = 0; i < N-1; i++){
        area += tri_area(points[i],points[i+1]);
    }
    area += tri_area(points[N-1], points[0]);
    return abs(area/2.);
}
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&N);
    double x, y;
    for(int i = 0; i < N; i++){
        scanf("%lf %lf",&x,&y);
        points.push_back(make_pair(x,y));
    }
    printf("%lf", find_area());
    return 0;
}