Approach: Stack of segments

def simplifyPath(path):
    st = []
    for part in path.split("/"):
        if part == "" or part == ".": continue
        if part == "..":
            if st: st.pop()
        else: st.append(part)
    return "/" + "/".join(st)

Complexity: O(n) time, O(n) space.

Full Solution — all languages

class Solution:
    def simplifyPath(self, s):
        st = []
        for part in s.split("/"):
            if part == "" or part == ".":
                continue
            if part == "..":
                if st: st.pop()
            else:
                st.append(part)
        return "/" + "/".join(st)

import java.util.*;
class Solution {
    public String simplifyPath(String s) {
        Deque<String> st = new ArrayDeque<>();
        for (String part : s.split("/")) {
            if (part.isEmpty() || part.equals(".")) continue;
            if (part.equals("..")) {
                if (!st.isEmpty()) st.pollLast();
            } else st.addLast(part);
        }
        StringBuilder sb = new StringBuilder();
        for (String p : st) sb.append('/').append(p);
        return sb.length() == 0 ? "/" : sb.toString();
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: string simplifyPath(string s) {
        vector<string> st;
        stringstream ss(s);
        string part;
        while (getline(ss, part, '/')) {
            if (part.empty() || part == ".") continue;
            if (part == "..") {
                if (!st.empty()) st.pop_back();
            } else st.push_back(part);
        }
        string res;
        for (auto& p : st) res += "/" + p;
        return res.empty() ? "/" : res;
    }
};

/**
 * @param {string} s
 * @return {string}
 */ var simplifyPath = function(s) {
    const st = [];
    for (const part of s.split('/')) {
        if (part === '' || part === '.') continue;
        if (part === '..') {
            if (st.length) st.pop();
        } else st.push(part);
    }
    return '/' + st.join('/');
};

Simplify Path

Example 1

Example 2

Example 3

Constraints