simplifyPaths.java

// Given an absolute path for a file (Unix-style), simplify it. Or in other words, convert it 
// to the canonical path.

// In a UNIX-style file system, a period . refers to the current directory. Furthermore, a double
//  period .. moves the directory up a level.

// Note that the returned canonical path must always begin with a slash /, and there must be only a 
// single slash / between two directory names. The last directory name (if it exists) must not end with 
// a trailing /. Also, the canonical path must be the shortest string representing the absolute path.

// Example 1:

// Input: "/home/"
// Output: "/home"
// Explanation: Note that there is no trailing slash after the last directory name.
// Example 2:

// Input: "/../"
// Output: "/"
// Explanation: Going one level up from the root directory is a no-op, as the root level is the highest level you can go.
// Example 3:

// Input: "/home//foo/"
// Output: "/home/foo"
// Explanation: In the canonical path, multiple consecutive slashes are replaced by a single one.
// Example 4:

// Input: "/a/./b/../../c/"
// Output: "/c"



//TC: O(N) to process the input stringß
//SC: O(N) for the stack to grow up to N

class Solution {
    public String simplifyPath(String path) {
        
        // Handle empty string
        if (path.isEmpty()) {
            return path;
        }
        // Initialize a stack
        Stack<String> stack = new Stack<String>();
        String[] components = path.split("/");
        
        // Split the input string on "/" as the delimiter
        // and process each portion one by one
        for (String directory : components) {
            
            // A no-op for a "." or an empty string
            if (directory.equals(".") || directory.isEmpty()) {
                continue;
            } else if (directory.equals("..")) {
                
                // If the current component is a "..", then
                // we pop an entry from the stack if it's non-empty
                if (!stack.isEmpty()) {
                    stack.pop();
                }
            } else {
                
                // Finally, a legitimate directory name, so we add it
                // to our stack
                stack.add(directory);
            }
        }
        // Stich together all the directory names together    
        StringBuilder result = new StringBuilder();
        for (String dir : stack) {
            result.append("/");
            result.append(dir);
        }
        
        return result.length() > 0 ? result.toString() : "/" ;
    }
}