leet-things

Personal LeetCode practice repo. Solutions in Java 21.

To make problems runnable straight from the IDE, this repo includes a small helper (LeetIO) that parses LeetCode-style bracket inputs ([2,7,11,15], [[1,2],[3,4]], "hello") directly from System.in, and pretty-prints results in the same notation LeetCode uses for expected outputs. Just paste the example block from the problem page into the run console and hit enter.

---

Project layout

src/
├── leet/
│   ├── LeetIO.java     input parser + pretty printer
│   ├── ListNode.java   LeetCode-standard singly linked list node
│   ├── TreeNode.java   LeetCode-standard binary tree node
│   └── Node.java       unified N-ary tree / graph node
└── solutions/
    └── TwoSum.java     usage pattern / template

Each problem lives in its own file under solutions/, with a main method that reads input via LeetIO, calls the solution method, and prints the result.

LeetIO

A Scanner(System.in) wrapper. Each next* call consumes one line and parses it as the requested LeetCode bracket form.

Read

Category	Methods	Example input
Scalar	nextInt, nextLong, nextDouble, nextBoolean, nextChar, nextString	42, 3.14, true, "hello"
1D array	nextIntArray, nextLongArray, nextDoubleArray, nextBooleanArray, nextCharArray, nextStringArray	[1,2,3], ["a","b"]
2D matrix	nextIntMatrix, nextLongMatrix, nextDoubleMatrix, nextCharMatrix, nextStringMatrix	[[1,2],[3,4]]
List	nextIntList, nextLongList, nextStringList, nextIntListList, nextStringListList	[1,2,3], [[1,2],[3]]
Structures	nextListNode, nextTreeNode, nextNAryNode, nextGraphNode	[1,2,3], [1,2,3,null,4], [1,null,3,2,4,null,5,6], [[2,4],[1,3]]

Empty arrays ([]), negatives, whitespace, and string escapes (\n, \t, \") are all accepted.

Print

print(...) / println(...) overloads cover every input type and emit LeetCode-style notation:

• Arrays / matrices / Lists: [1,2,3], [[1,2],[3,4]] (compact, single line)
• ListNode: walked to end → [1,2,3]
• TreeNode: BFS level order with null, trailing nulls trimmed → [1,2,3,null,4]
• N-ary Node: printNAry() → [1,null,3,2,4,null,5,6]
• Graph Node: printGraph() → adjacency list [[2,4],[1,3],...]
• Debug helper: printPretty(int[][]) — multi-line indented matrix

Adding a solution

1. Create a new file under src/solutions/ (e.g. Solution001.java)
2. Write the solution method
3. In main, read input with LeetIO, call the method, print with io.println(...)
4. Run the class in IntelliJ (Ctrl+Shift+F10 on the file)
5. Paste the LeetCode example into the run console

solutions/TwoSum.java is the template:

package solutions;

import leet.LeetIO;

public class TwoSum {
    public int[] twoSum(int[] nums, int target) { /* ... */ }

    public static void main(String[] args) {
        LeetIO io = new LeetIO();
        int[] nums = io.nextIntArray();
        int target = io.nextInt();
        io.println(new TwoSum().twoSum(nums, target));
    }
}

Input:

[2,7,11,15]
9

Output:

[0,1]

Environment

• Java 21 (Temurin)
• IntelliJ IDEA
• No build tool — src/ is the source root, zero dependencies