Codeforces 27D（二分染色）

要点

• 将边作为染色，如果交叉则异色

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

int n, m;
int a[101], b[101], c[101];

int main() {
	scanf("%d %d", &n, &m);
	for (int i = 1; i <= m; i++) {
		scanf("%d %d", &a[i], &b[i]);
		if (a[i] > b[i])
			swap(a[i], b[i]);
		c[i] = -1;
	}

	function<void(int, int)> dfs = [&](int u, int color) {
		if (c[u] != -1 && c[u] != color) {
			puts("Impossible"); exit(0);
		} else if (c[u] == -1) {
			c[u] = color;
			for (int i = 1; i <= m; i++) {
				if (a[i] < a[u] && b[i] > a[u] && b[i] < b[u])
					dfs(i, color ^ 1);
				if (a[i] > a[u] && a[i] < b[u] && b[i] > b[u])
					dfs(i, color ^ 1);
			}
		}
	};

	for (int i = 1; i <= m; i++)
		if (c[i] < 0)
			dfs(i, 0);
	for (int i = 1; i <= m; i++)
		putchar(c[i] ? 'i' : 'o');
}