Bài toán
Giải phương trình bậc 3
Code này của Nguyễn Tiến Trung Kiên
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
#define float long double
float a, b, c, d;
vector<float> Result;
float h(float x) { return a * x * x * x + b * x * x + c * x + d; }
int sign(float x) {
if (fabs(x) <= 1e-9) return 0;
return x > 0 ? 1 : -1;
}
void decide(float x) { Result.push_back(x); }
bool solve(float a, float b, float c, float &x1, float &x2) {
if (sign(a) == 0) {
if (sign(b) == 0) return false;
x1 = x2 = -c / b;
return true;
}
float d = b * b - 4 * a * c;
if (sign(d) == 0) {
x1 = x2 = -b / (2 * a);
return true;
} else if (sign(d) > 0) {
x1 = (-b - sqrt(d)) / (2 * a);
x2 = (-b + sqrt(d)) / (2 * a);
if (x1 > x2) swap(x1, x2);
return true;
}
return false;
}
void search1(float ll, float rr) {
if (::isinf(ll)) {
ll = rr - 100;
while (sign(h(ll)) > 0) ll -= (rr - ll);
}
if (::isinf(rr)) {
rr = ll + 100;
while (sign(h(rr)) < 0) rr += (rr - ll);
}
//printf("search1(%Lg, %Lg)\n", ll, rr);
if (sign(h(ll)) == 0) return decide(ll);
if (sign(h(rr)) == 0) return decide(rr);
if (sign(h(ll)) > 0 || sign(h(rr)) < 0) return;
float mm = (ll + rr) / 2;
for (int i = 1; i <= 100; i++) {
h(mm) > 0 ? rr = mm : ll = mm;
mm = (ll + rr) / 2;
}
return decide(mm);
}
void search2(float ll, float rr) {
//printf("search2(%g, %g)\n", ll, rr);
if (sign(h(ll)) == 0) return decide(ll);
if (sign(h(rr)) == 0) return decide(rr);
if (sign(h(ll)) < 0 || sign(h(rr)) > 0) return;
float mm = (ll + rr) / 2;
for (int i = 1; i <= 100; i++) {
h(mm) < 0 ? rr = mm : ll = mm;
mm = (ll + rr) / 2;
}
return decide(mm);
}
int main() {
while (cin >> a >> b >> c >> d) {
if (sign(a) == 0) {
float x1, x2;
if (solve(b, c, d, x1, x2)) {
decide(x1);
decide(x2);
}
} else {
if (a < 0) {
a = -a, b = -b, c = -c, d = -d;
}
float x1, x2;
if (solve(3 * a, 2 * b, c, x1, x2)) {
search1(-1.0 / 0.0, x1);
search2(x1, x2);
search1(x2, +1.0 / 0.0);
} else {
search1(-1.0 / 0.0, 0);
search1(0, +1.0 / 0.0);
}
}
sort(Result.begin(), Result.end());
Result.erase(unique(Result.begin(), Result.end()), Result.end());
cout.precision(6);
for (float x : Result)
cout << (fabs(x) <= 1e-15 ? 0.0 : x) << endl;
Result.clear();
}
}