Language: Java

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author maheshwari29
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        GLongestPath solver = new GLongestPath();
        solver.solve(1, in, out);
        out.close();
    }

    static class GLongestPath {
        static ArrayList<Integer> arr;
        static boolean visited[];
        static int dp[];

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int i, j;
            int n = in.ni();
            int m = in.ni();
            arr = new ArrayList[n];
            visited = new boolean[n];
            dp = new int[n];
            for (i = 0; i < n; i++)
                arr[i] = new ArrayList<Integer>();
            for (i = 0; i < m; i++)
                arr[in.ni() - 1].add(in.ni() - 1);
            for (i = 0; i < n; i++) {
                if (!visited[i]) {   //System.out.println("i-="+i);
                    dfs(i);
                    //System.out.print("------------");
                }
            }
            int max = 0;
            for (i = 0; i < n; i++)
                max = Math.max(dp[i], max);
            out.println(max);
            //out.println(dp);
        }

        static void dfs(int a) {   //System.out.println(a);
            visited[a] = true;
            for (int c : arr[a]) {
                if (!visited[c])
                    dfs(c);
                dp[a] = Math.max(dp[c] + 1, dp[a]);
            }
            return;
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int ni() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }
}



Compiler output:
Main.java:40: error: incompatible types: ArrayList[] cannot be converted to ArrayList<Integer>
            arr = new ArrayList[n];
                  ^
Main.java:44: error: array required, but ArrayList<Integer> found
                arr[i] = new ArrayList<Integer>();
                   ^
Main.java:46: error: array required, but ArrayList<Integer> found
                arr[in.ni() - 1].add(in.ni() - 1);
                   ^
Main.java:62: error: array required, but ArrayList<Integer> found
            for (int c : arr[a]) {
                            ^
4 errors