Solution.java

class Solution {
    public String simplifyPath(String path) {
        List<String> dirs = new ArrayList<>();
        int dirStart = 0, len = path.length();
        while (dirStart < len) {
            while (dirStart < len && path.charAt(dirStart) == '/') dirStart++;
            int dirEnd = dirStart;
            while (dirEnd < len && path.charAt(dirEnd) != '/') dirEnd++;
            String dir = path.substring(dirStart, dirEnd);
            if (!".".equals(dir)) {
                if ("..".equals(dir)) {
                    if (! dirs.isEmpty()) dirs.remove(dirs.size() - 1);
                } else if (dir.length() > 0) {
                    dirs.add(dir);
                }
            }
            dirStart = ++dirEnd;
        }
        StringBuilder sb = new StringBuilder("/");
        for (int i = 0; i < dirs.size(); i++) {
            if (i == dirs.size() - 1) sb.append(dirs.get(i));
            else sb.append(dirs.get(i)).append("/");
        }
        return sb.toString();
    }
}