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Problem solution · Java

Delete Duplicate Folders in System

Delete Duplicate Folders in System: a Java solution using sorting and greedy selection. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Sorting and greedy selection
Source
walkccc LeetCode Solutions
Length
71 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Sorting and greedy selection

For Delete Duplicate Folders in System, the implementation first exposes a useful order, then scans that order while making locally justified choices.

  1. Choose the key that reveals the greedy or grouping structure.
  2. Sort the relevant records by that key.
  3. Scan in order, maintaining the invariant that makes each local choice safe.

Code notes

  • 71 lines of Java from the credited upstream file 1948.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 7 loop blocks detected.

Complexity

Sorting is typically the dominant term unless the subsequent scan uses a more expensive nested operation.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeDelete Duplicate Folders in System · JavaJava
Use this to learn the idea, then write your own version.
class TrieNode {  public Map<String, TrieNode> children = new HashMap<>();  public boolean deleted = false;} class Solution {  public List<List<String>> deleteDuplicateFolder(List<List<String>> paths) {    List<List<String>> ans = new ArrayList<>();    Map<String, List<TrieNode>> subtreeToNodes = new HashMap<>();     Collections.sort(paths,                     Comparator.<List<String>>comparingInt(List::size).thenComparing((a, b) -> {                       for (int i = 0; i < Math.min(a.size(), b.size()); ++i) {                         final int c = a.get(i).compareTo(b.get(i));                         if (c != 0)                           return c;                       }                       return 0;                     }));     for (List<String> path : paths) {      TrieNode node = root;      for (final String s : path) {        node.children.putIfAbsent(s, new TrieNode());        node = node.children.get(s);      }    }     buildSubtreeToRoots(root, subtreeToNodes);     for (List<TrieNode> nodes : subtreeToNodes.values())      if (nodes.size() > 1)        for (TrieNode node : nodes)          node.deleted = true;     constructPath(root, new ArrayList<>(), ans);    return ans;  }   private TrieNode root = new TrieNode();   private StringBuilder buildSubtreeToRoots(TrieNode node,                                            Map<String, List<TrieNode>> subtreeToNodes) {    StringBuilder sb = new StringBuilder("(");    for (final String s : node.children.keySet()) {      TrieNode child = node.children.get(s);      sb.append(s).append(buildSubtreeToRoots(child, subtreeToNodes));    }    sb.append(")");    final String subtree = sb.toString();    if (!subtree.equals("()")) {      subtreeToNodes.putIfAbsent(subtree, new ArrayList<>());      subtreeToNodes.get(subtree).add(node);    }    return sb;  }   private void constructPath(TrieNode node, List<String> path, List<List<String>> ans) {    for (final String s : node.children.keySet()) {      TrieNode child = node.children.get(s);      if (!child.deleted) {        path.add(s);        constructPath(child, path, ans);        path.remove(path.size() - 1);      }    }    if (!path.isEmpty())      ans.add(new ArrayList<>(path));  }}

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