1487. Making File Names Unique

Time: O(n·m)

Space: O(n·m)

Approach

Dictionary tracks the next suffix to try for each name; increment suffix until a fresh name is found.

Key Techniques

Hash tables provide O(1) average-case lookup, insert, and delete. They are the go-to tool for counting frequencies, detecting complements (Two Sum pattern), and caching seen values. In C#, use Dictionary<K,V> for maps and HashSet<T> for membership checks.

String

String problems range from simple character counting to complex pattern matching. Common approaches include two pointers, sliding window, prefix hashing, and the KMP algorithm. In C#, strings are immutable — use StringBuilder for efficient concatenation inside loops.

1487.cs

// Approach: Dictionary tracks the next suffix to try for each name; increment suffix until a fresh name is found.
// Time: O(n·m) Space: O(n·m)

public class Solution
{
    public string[] GetFolderNames(string[] names)
    {
        string[] ans = new string[names.Length];
        Dictionary<string, int> nameToSuffix = new Dictionary<string, int>();

        for (int i = 0; i < names.Length; ++i)
        {
            string name = names[i];
            if (nameToSuffix.ContainsKey(name))
            {
                int suffix = nameToSuffix[name];
                string newName = GetName(name, ++suffix);
                while (nameToSuffix.ContainsKey(newName))
                    newName = GetName(name, ++suffix);
                nameToSuffix[name] = suffix;
                nameToSuffix[newName] = 0;
                ans[i] = newName;
            }
            else
            {
                nameToSuffix[name] = 0;
                ans[i] = name;
            }
        }

        return ans;
    }

    private string GetName(string name, int suffix)
    {
        return name + "(" + suffix.ToString() + ")";
    }
}

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