Language: Java

import java.io.*;
import java.util.*;
import java.math.*;
// import java.awt.Point;
 
public class Main {
    InputStream is;
    PrintWriter out;
    String INPUT = "";
 
    static int mod = 1_000_000_007;
    // int mod = 998244353;
    // long inf = Long.MAX_VALUE/2;
    int inf = Integer.MAX_VALUE/2;

    ArrayList<Integer>[] edge;
    int[] num, num2;
    boolean[] seen;
    HashSet<Long> map = new HashSet<>();
    void solve(){
        int n = ni();
        int m = ni();
        edge = new ArrayList[n];
        for(int i = 0; i < n; i++){
            edge[i] = new ArrayList<Integer>();
        }
        num = new int[n];
        num2 = new int[n];
       
        for(int i = 0; i < m; i++){
            int s = ni()-1;
            int t = ni()-1;
            long ret = s*1000000000l + t;
            map.add(ret);
            edge[s].add(t);
            edge[t].add(s);
            num[s]++;
            num[s] = num[s]%2;
        }
        if((m%2)!=0){
            out.println(-1);
            return;
        }
        seen = new boolean[n];
        dfs(0); 
        int[] ans = new int[n];
        for(long i : map){
            long s = i/1000000000l;
            long t = i%1000000000l;
            out.println((s+1)+" "+(t+1));
            ans[(int)s]++;
        } 
        for(int i = 0; i < n; i++){
            if(ans[i]%2!=0){
                out.println(i);
            }
        }
    }

    int dfs(int cur){
        seen[cur] = true;
        int ret = num[cur];
        System.err.println("orig: "+cur+" "+ret);
        for(int e : edge[cur]){
            if(seen[e]) continue;
            int tmp = dfs(e);
            if(tmp%2==1){
                out.println("flip: "+cur+" "+e);
                long res = cur*1000000000l + e;
                long res2 = e*1000000000l + cur;
                if(map.containsKey(res)){
                    map.remove(res);
                    map.add(res2);
                }
                else{
                    map.remove(res2);
                    map.add(res);
                }
                ret = 1 - ret;  
            }
        }
        // out.println("result: "+cur+" "+ret);
        return ret%2;
    }

    void run() throws Exception
    {
        is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        solve();
        out.flush();
        if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
    }
    
    public static void main(String[] args) throws Exception { new Main().run(); }
    
    private byte[] inbuf = new byte[1024];
    private int lenbuf = 0, ptrbuf = 0;
    
    private int readByte()
    {
        if(lenbuf == -1)throw new InputMismatchException();
        if(ptrbuf >= lenbuf){
            ptrbuf = 0;
            try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
            if(lenbuf <= 0)return -1;
        }
        return inbuf[ptrbuf++];
    }
    
    private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
    private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
    
    private double nd() { return Double.parseDouble(ns()); }
    private char nc() { return (char)skip(); }
    
    private String ns()
    {
        int b = skip();
        StringBuilder sb = new StringBuilder();
        while(!(isSpaceChar(b) && b != ' ')){
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    
    private char[] ns(int n)
    {
        char[] buf = new char[n];
        int b = skip(), p = 0;
        while(p < n && !(isSpaceChar(b))){
            buf[p++] = (char)b;
            b = readByte();
        }
        return n == p ? buf : Arrays.copyOf(buf, p);
    }
    
    private char[][] nm(int n, int m)
    {
        char[][] map = new char[n][];
        for(int i = 0;i < n;i++)map[i] = ns(m);
        return map;
    }
    
    private int[] na(int n)
    {
        int[] a = new int[n];
        for(int i = 0;i < n;i++)a[i] = ni();
        return a;
    }
    
    private int ni()
    {
        int num = 0, b;
        boolean minus = false;
        while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
        if(b == '-'){
            minus = true;
            b = readByte();
        }
        
        while(true){
            if(b >= '0' && b <= '9'){
                num = num * 10 + (b - '0');
            }else{
                return minus ? -num : num;
            }
            b = readByte();
        }
    }
    
    private long nl()
    {
        long num = 0;
        int b;
        boolean minus = false;
        while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
        if(b == '-'){
            minus = true;
            b = readByte();
        }
        
        while(true){
            if(b >= '0' && b <= '9'){
                num = num * 10 + (b - '0');
            }else{
                return minus ? -num : num;
            }
            b = readByte();
        }
    }
    
    private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); }
 
}


Compiler output:
Main.java:71: error: cannot find symbol
                if(map.containsKey(res)){
                      ^
  symbol:   method containsKey(long)
  location: variable map of type HashSet<Long>
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error