UVALive - 3211 Now or later （【2-SAT】+【二分】）

題目鏈接：https://cn.vjudge.net/problem/UVALive-3211

【AC代碼】

#include<cstdlib>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<stack>
#include<set>
using namespace std;
#define LL long long

const int maxn = 50000 + 5;

struct TwoSAT
{
    int n;
    vector<int>G[maxn*2];
    bool mark[maxn*2];
    int s[maxn*2], c;
    bool dfs(int x)
    {
        if(mark[x^1])return false;
        if(mark[x])return true;
        mark[x] = true;
        s[c++] = x;
        for(int i = 0; i < G[x].size(); i++)
        {
            if(!dfs(G[x][i]))return false;
        }
        return true;
    }
    void init(int n)
    {
        this->n = n;
        for(int i = 0; i < n*2; i++)
        {
            G[i].clear();
        }
        memset(mark, 0, sizeof(mark));
    }
    void add_caluse(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 solve()
    {
        for(int i = 0; i < n*2; i+=2)
        {
            if(!mark[i]&&!mark[i+1])
            {
                c = 0;
                if(!dfs(i))
                {
                    while(c > 0)
                    {
                        mark[s[--c]] = false;
                    }
                    if(!dfs(i+1))
                    {
                        return false;
                    }
                }
            }
        }
        return true;
    }
};

TwoSAT solver;
int n,T[maxn][2];
bool test(int diff)
{
    solver.init(n);
    for(int i = 0; i < n; i++)
    {
        for(int a = 0; a < 2; a++)
        {
            for(int j = i+1; j < n; j++)
            {
                for(int b = 0; b < 2; b++)
                {
                    if(abs(T[i][a]-T[j][b]) < diff)
                    {
                        solver.add_caluse(i, a^1, j, b^1);
                    }
                }
            }
        }
    }
    return solver.solve();
}

int main()
{
    while(~scanf("%d", &n))
    {
        if(n == 0)break;
        int L = 0, R = 0;
        for(int i = 0; i < n; i++)for(int a = 0; a < 2; a++)
        {
            scanf("%d",&T[i][a]);
            R = max(R, T[i][a]);
        }
        while(L < R)
        {
            int M = L + (R - L + 1)/2;
            if(test(M))L = M;
            else R = M - 1;
        }
        printf("%d\n",L);
    }
    return 0;
}