[BOJ] 11664 선분과 점(삼분탐색).cpp

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

double dist(double x1, double y1, double z1, double x2, double y2, double z2) {
    return sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1)+(z2-z1)*(z2-z1));
}

int main() {
    int x1, x2, x3, y1, y2, y3, z1, z2, z3;
    cin >> x1 >> y1 >> z1 >> x2 >> y2 >> z2 >> x3 >> y3 >> z3;
    double dx = x2-x1;
    double dy = y2-y1;
    double dz = z2-z1;
    double left = 0.0;
    double right = 1.0;
    double m = 0;
    while (true) {
        if (abs(right-left) < 1e-9) { // 소수 9자리 수까지 비교
            m = (left+right)/2;
            break;
        }
        double m1 = left + (right-left)/3;
        double m2 = right - (right-left)/3;
        double m1x = x1 + m1*dx;
        double m1y = y1 + m1*dy;
        double m1z = z1 + m1*dz;
        double m2x = x1 + m2*dx;
        double m2y = y1 + m2*dy;
        double m2z = z1 + m2*dz;
        double d1 = dist(m1x, m1y, m1z, x3, y3, z3);
        double d2 = dist(m2x, m2y, m2z, x3, y3, z3);
        if (d1 > d2) {
            left = m1;
        }
        else {
            right = m2;
        }
    }
    double x0 = x1 + m*dx;
    double y0 = y1 + m*dy;
    double z0 = z1 + m*dz;
    double ans = dist(x0, y0, z0, x3, y3, z3);
    cout << fixed << setprecision(10) << ans << endl; // 소수 10자리까지 출력
    return 0;
}