71. Simplify Path

Stack

class Solution {
    public String simplifyPath(String path) {
        Stack<String> stack = new Stack<>();
        int n = path.length();
        int i = 0;

        while (i < n) {
            // skip multiple '/'
            while (i < n && path.charAt(i) == '/') {
                i++;
            }
            if (i >= n) break;

            // extract the path name
            int start = i;
            while (i < n && path.charAt(i) != '/') {
                i++;
            }
            String part = path.substring(start, i);

            // process the path name
            if (part.equals("..")) {
                if (!stack.isEmpty()) {
                    stack.pop();
                }
            } else if (!part.equals(".") && !part.isEmpty()) {
                stack.push(part);
            }
        }

        // construct the result
        if (stack.isEmpty()) return "/";

        StringBuilder result = new StringBuilder();
        for (String dir : stack) {
            result.append("/").append(dir);
        }

        return result.toString();
    }
}

Remarks:

1. TC: $O(n)$, SC: $O(n)$.