二分图（染色法）

import java.util.Scanner;

public class Main {

    private static int index = 0;

    private static int[] lastEdge;

    private static int[] end;

    private static int[] previousEdge;

    private static int[] color;

    private static int n;

    private static int m;

    private static void addEdge(int a, int b) {
        index++;
        end[index] = b;
        previousEdge[index] = lastEdge[a];
        lastEdge[a] = index;
    }

    private static void init(int n, int m) {
        Main.n = n;
        Main.m = m;
        Main.index = 0;
        end = new int[2 * m + 1];
        lastEdge = new int[n + 1];
        previousEdge = new int[2 * m + 1];
        color = new int[n + 1];
    }

    private static boolean dfs(int x, int c) {
        color[x] = c;

        for (int edge = lastEdge[x]; edge != 0; edge = previousEdge[edge]) {

            int b = end[edge];

            if (color[b] == 0) {
                if (! dfs(b, 3 - c)) {
                    return false;
                }
            } else if (color[b] == c) {
                return false;
            }
        }

        return true;

    }

    private static boolean isBipartiteGraph() {

        boolean flag = true;

        for (int i = 1;i <= n; ++ i) {
            if (color[i] == 0) {
                if (! dfs(i, 1)) {
                    flag = false;
                    break;
                }
            }
        }

        return flag;
    }

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);

        while (in.hasNext()) {

            int n = in.nextInt();
            int m = in.nextInt();

            init(n, m);

            while (m -- > 0) {
                int a = in.nextInt();
                int b = in.nextInt();
                addEdge(a, b);
                addEdge(b, a);
            }

            System.out.println(isBipartiteGraph());
        }
    }
}