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());
}
}
}