2-SAT解法模板

摘自大白书的算法模板如下

const int maxn = 1050;

struct TwoSAT{
    int n;
    vector<int> g[maxn * 2];
    bool mark[maxn * 2];//x[2*u]被标记代表u为假 x[2*u+1]被标记代表u为真
    int s[maxn * 2], top;

    void init(int n) {
        this->n = n;
        for (int i = 0; i < n * 2; ++i) g[i].clear();
        memset(mark, 0, sizeof(mark));
    }

    //x = xval or y = yval
    void add_clause(int x, int xval, int y, int yval) {
        x = x * 2 + xval;
        y = y * 2 + yval;
        g[x^1].push_back(y);
        g[y^1].push_back(x);
    }

    bool dfs(int x) {
        if (mark[x^1]) return false;
        if (mark[x]) return true;
        mark[x] = true;
        s[top++] = x;
        for (int i = 0; i < g[x].size(); ++i) {
            if (!dfs(g[x][i])) return false;
        }
        return true;
    }

    bool solve() {
        for (int i = 0; i < n * 2; i += 2) {//注意是i+=2，很容易写成i++
            if (!mark[i] && !mark[i + 1]) {
                top = 0;
                if (!dfs(i)) {
                    while (top > 0) mark[s[--top]] = false;
                    if (!dfs(i + 1)) return false;
                }
            }
        }
        return true;
    }
};