SimplifyPath.java

package by.andd3dfx.collections;

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * <pre>
 * <a href="https://leetcode.com/problems/simplify-path/description/">Task description</a>
 *
 * You are given an absolute path for a Unix-style file system, which always begins with a slash '/'. Your task is to transform this absolute path into its simplified canonical path.
 *
 * The rules of a Unix-style file system are as follows:
 *
 *     A single period '.' represents the current directory.
 *     A double period '..' represents the previous/parent directory.
 *     Multiple consecutive slashes such as '//' and '///' are treated as a single slash '/'.
 *     Any sequence of periods that does not match the rules above should be treated as a valid directory or file name. For example, '...' and '....' are valid directory or file names.
 *
 * The simplified canonical path should follow these rules:
 *
 *     The path must start with a single slash '/'.
 *     Directories within the path must be separated by exactly one slash '/'.
 *     The path must not end with a slash '/', unless it is the root directory.
 *     The path must not have any single or double periods ('.' and '..') used to denote current or parent directories.
 *
 * Return the simplified canonical path.
 *
 * Example 1:
 * Input: path = "/home/"
 * Output: "/home"
 * Explanation:
 * The trailing slash should be removed.
 *
 * Example 2:
 * Input: path = "/home//foo/"
 * Output: "/home/foo"
 * Explanation:
 * Multiple consecutive slashes are replaced by a single one.
 *
 * Example 3:
 * Input: path = "/home/user/Documents/../Pictures"
 * Output: "/home/user/Pictures"
 * Explanation:
 * A double period ".." refers to the directory up a level (the parent directory).
 *
 * Example 4:
 * Input: path = "/../"
 * Output: "/"
 * Explanation:
 * Going one level up from the root directory is not possible.
 *
 * Example 5:
 * Input: path = "/.../a/../b/c/../d/./"
 * Output: "/.../b/d"
 * Explanation:
 * "..." is a valid name for a directory in this problem.
 * </pre>
 *
 * @see <a href="https://youtu.be/ZONpIPMSNPk">Video solution</a>
 */
public class SimplifyPath {

    public static String simplifyPath(String path) {
        Deque<String> stack = new ArrayDeque<>();

        path = path.replaceAll("/+", "/");
        StringBuilder accumulator = new StringBuilder();
        var chars = path.toCharArray();
        var i = 1;
        while (i < chars.length) {
            if (chars[i] == '/') {
                processAccumulated(accumulator, stack);
                accumulator = new StringBuilder();
            } else {
                accumulator.append(chars[i]);
            }
            i++;
        }
        processAccumulated(accumulator, stack);

        return "/" + String.join("/", stack.reversed());
    }

    private static void processAccumulated(StringBuilder accumulator, Deque<String> stack) {
        var accumulated = accumulator.toString();
        if (accumulated.equals("..") && !stack.isEmpty()) {
            stack.pop();
        } else if (!accumulated.matches("\\.{1,2}") && !accumulated.isEmpty()) {
            stack.push(accumulated);
        }
    }
}