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The Berkeley parser analyzes the grammatical structure of natural language using probabilistic context-free grammars (PCFGs).

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package edu.berkeley.nlp.syntax;

import java.util.List;

public class TreePath {

	public enum Direction {
		UP,
		DOWN,
		DOWN_LEFT,
		DOWN_RIGHT;
	}

	public static class Transition {

		private Tree fromNode;
		private Tree toNode;
		private Direction direction;

		public Transition(Tree fromNode, Tree toNode, Direction direction) {
			this.fromNode = fromNode;
			this.toNode = toNode;
			this.direction = direction;
		}

		public Tree getFromNode() {
			return fromNode;
		}

		public Tree getToNode() {
			return toNode;
		}

		public Direction getDirection() {
			return direction;
		}
	}

	private Tree startNode;
	private Tree endNode;
	private List> transitions;

	public TreePath(List> transitions) {
		if (transitions == null || transitions.size() == 0) {
			throw new IllegalArgumentException("Cannot have empty transitions list");
		}
		this.transitions = transitions;
		startNode = transitions.get(0).fromNode;
		endNode = transitions.get(transitions.size()-1).toNode;
	}

	public Tree getStartNode() {
		return startNode;
	}

	public Tree getEndNode() {
		return endNode;
	}

	public List> getTransitions() {
		return transitions;
	}

	private String cacheStr = null;

	@Override
	public String toString() {
		if (cacheStr == null) {
			StringBuilder sb = new StringBuilder();
			sb.append("[ ");
			for (Transition transition : transitions) {
				sb.append(transition.fromNode.getLabel() + " " + transition.direction + " ");
			}
			sb.append(endNode.getLabel() + " ]");
			cacheStr = sb.toString();
		}
		return cacheStr;
	}
}