紫书 例题8-14 UVa 1607 （二分）

题意非常难理解……

#include<cstdio>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;

const int MAXN = 212345;
struct node
{
	int a, b, w;
}g[MAXN];
int n, m;

int output(int k)
{
	REP(i, 1, m + 1)
	{
		int a = g[i].a;
		int b = g[i].b;
		int va = a < 0 ? -a > k : g[a].w;
		int vb = b < 0 ? -b > k : g[b].w;
		g[i].w = !(va && vb);
	}
	return g[m].w;
}

int solve(int vn)
{
	int l = 0, r = n + 1;
	while(l + 1 < r)
	{
		int mid = (l + r) >> 1;
		if(output(mid) == vn) r = mid;
		else l = mid;
	}
	return r;
}

int main()
{
	int T;
	scanf("%d", &T);
	
	while(T--)
	{
		scanf("%d%d", &n, &m);
		REP(i, 1, m + 1) scanf("%d%d", &g[i].a, &g[i].b);
		
		int v0 = output(0);
		int vn = output(n);
		if(v0 == vn)
		{
			REP(i, 1, n + 1) putchar('0');
			puts(""); continue;
		}
		
		int x = solve(vn);
		REP(i, 1, x) putchar('0');
		putchar('x');
		REP(i, x + 1, n + 1) putchar('1');
		puts("");
	}
	
	return 0;	
}