简化路径

import java.util.*;

public class Solution {
    public String simplifyPath(String path) {
        if (path == null) {
            return "/";
        }
        String[] subStrs = path.split("/");
        List<String> res = new ArrayList();
        for (int i = 0;i < subStrs.length;i++) {
            if (subStrs[i].equals("") || subStrs[i].equals(".")) {
                continue;
            } else if (subStrs[i].equals("..")) {
                if (!res.isEmpty()) {
                    res.remove(res.size() - 1);
                }
            } else {
                res.add(subStrs[i]);
            }
        }
        StringBuilder sb = new StringBuilder();
        for (String s : res) {
            sb.append("/");
            sb.append(s);
        }
        return sb.toString().isEmpty() ? "/" : sb.toString();
    }
}

public String simplifyPath(String path) {
        Stack<String> input = new Stack<>();
        Stack<String> result = new Stack<>();
        String[] split = path.split("/");
        
        for(String s : split) if(s.length() > 0) input.push(s);    
        for(String s : input){
            if(s.equals("..")) {
                if(!result.isEmpty()) result.pop();
            }
            else if(s.equals(".")) continue;
            else result.push(s);
        }
        
        return "/" + String.join("/", result);
    }

The main idea is to push to the stack every valid file name (not in {"",".",".."}), popping only if there's smth to pop and we met "..". I don't feel like the code below needs any additional comments.

public String simplifyPath(String path) {
    Deque<String> stack = new LinkedList<>(); Set<String> skip = new HashSet<>(Arrays.asList("..",".",""));
    for (String dir : path.split("/")) { if (dir.equals("..") && !stack.isEmpty()) stack.pop(); else if (!skip.contains(dir)) stack.push(dir);
    } String res = "";
    for (String dir : stack) res = "/" + dir + res; return res.isEmpty() ? "/" : res;
}