Note -「模板」高斯消元

#include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;

double Abs(double x) { return x < 0 ? -x : x; }
double Max(double x, double y) { return x > y ? x : y; }
double Min(double x, double y) { return x < y ? x : y; }

int read() {
    int x = 0, k = 1;
    char s = getchar();
    while (s < '0' || s > '9') {
        if (s == '-')
            k = -1;
        s = getchar();
    }
    while ('0' <= s && s <= '9') {
        x = (x << 3) + (x << 1) + (s ^ 48);
        s = getchar();
    }
    return x * k;
}

void write(int x) {
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    if (x > 9)
        write(x / 10);
    putchar(x % 10 + '0');
}

void print(int x, char c) {
    write(x);
    putchar(c);
}

const int MAXN = 5e2 + 5;
const int MAXM = 3e5 + 5;

struct Elimination {
    bool free[MAXN];
    int n, m, rk, opt;
    double a[MAXN][MAXN], x[MAXN], eps;
    Elimination() { eps = 1e-12; }
    Elimination(int N, int M) {
        n = N;
        m = M;
    }
    double Abs(double x) { return x < eps ? -x : x; }
    void Swap(double &x, double &y) {
        double t = x;
        x = y;
        y = t;
    }

    void clear() {
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= m; j++) a[i][j] = 0;
    }

    void calc() {
        int r = 1, c = 1;
        for (; r <= n && c <= m; r++, c++) {
            int pos = r;
            for (int i = r + 1; i <= n; i++)
                if (Abs(a[i][c]) > Abs(a[pos][c]))
                    pos = i;
            if (Abs(a[pos][c]) < eps) {
                r--;
                continue;
            }
            if (pos != r)
                for (int i = c; i <= m; i++) Swap(a[r][i], a[pos][i]);
            double t;
            for (int i = 1; i <= n; i++)
                if (i != r && Abs(a[i][c]) > eps) {
                    t = a[i][c] / a[r][c];
                    for (int j = m; j >= c; j--) a[i][j] -= t * a[r][j];
                }
        }
        rk = r;
    }

    void check() {
        opt = 1;
        for (int i = 1; i <= n; i++)
            if (Abs(a[i][i]) < eps && Abs(a[i][m]) > eps) {
                opt = -1;
                return;
            }
        for (int i = 1; i <= n; i++)
            if (Abs(a[i][i]) < eps && Abs(a[i][m]) < eps) {
                free[i] = true;
                opt = 0;
            } else
                x[i] = a[i][m] / a[i][i];
    }
};