[Java] Challenge 0 to 10 (Unreviewed)

challenge_0/java/jdfurlan/README.md

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+# HelloWorld
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+Make it unnecessarily slow!
+
+### 2. How to compile and run this code
+
+```
+javac HelloWorld.java
+java HelloWorld
+```
+
+### 3. How this program works
+
+Simply run it and you'll see "Hello, World!"

challenge_0/java/jdfurlan/src/HelloWorld.java

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+
+public class HelloWorld {
+
+	public static void main(String[] args) {
+		char[] hw = "Hello, World!".toCharArray();
+		for (char c : hw) {
+			System.out.print(c);
+		}
+
+	}
+
+}

challenge_1/java/jdfurlan/README.md

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+# ReverseAString
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+Don't use built-in methods, just primitives and arrays
+
+### 2. How to compile and run this code
+
+```
+javac ReverseAString.java
+java ReverseAString
+```
+
+### 3. How this program works
+
+Takes user input and converts to a character array
+Track first and last values, store one in a temp variable, then swap
+increment and decrement the positions and continue to swap. 
+For odd length strings it just leaves the middle value in place, since no swap needed

challenge_1/java/jdfurlan/src/ReverseAString.java

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+
+import java.util.Scanner;
+
+public class ReverseAString {
+
+	public static void main(String[] args) {
+		Scanner sc = new Scanner(System.in);
+		char[] input = sc.next().toCharArray();
+		int j = input.length - 1;
+		for (int i = 0; i < input.length / 2; i++, j--) {
+			char temp = input[j];
+			input[j] = input[i];
+			input[i] = temp;
+		}
+		System.out.println(String.valueOf(input));
+		sc.close();
+	}
+}

challenge_10/java/jdfurlan/README.md

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+# ValidClosers
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+Struggled for a while and looked at zmiller91's approach and derived something very similar
+Once again his logic helped me learn much on this one!
+
+### 2. How to compile and run this code
+
+
+```
+javac ValidClosers.java
+java ValidClosers
+```
+
+### 3. How this program works
+
+Track openers "(" "{" or "[" in one list, and closers in another.	
+This problem pretty much requires a stack, which is last in first out. 
+Put all the openers on the stack, which is the same but reverse order that closers
+should be in the rest of the string.
+
+eg. if we actually used two stacks, they would look like this:
+```
+openers = (, (, (
+closers = ), ), )
+```
+
+So we continue iterating through the closers from "first to last", and compare "last to first" in the openers stack.
+If we have a match, it's safe to pop it off the stack and check the next pair. If the stack's empty we know they all matched.
+

challenge_10/java/jdfurlan/src/ValidClosers.java

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+
+import java.util.List;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Scanner;
+import java.util.Stack;
+/**
+ * 
+ * @author jdfurlan but zmiller91's comments and solution helped immensely!
+ *
+ */
+public class ValidClosers {
+
+	private static boolean checkClosers(String nextLine) {
+		if (nextLine.length() == 0)
+			return true;
+		List<Character> openers = Arrays.asList('{', '[', '(');
+		List<Character> closers = Arrays.asList('}', ']', ')');
+		Stack<Character> stack = new Stack<Character>();
+		for (char c : nextLine.toCharArray()) {
+			// if we found an opener just put it at the top of the stack
+			if (openers.contains(c)) {
+				stack.push(c);
+			}
+			else if (closers.contains(c)) {
+				// the stack can't be empty when looking at a closer, otherwise
+				// they don't match
+				if (!stack.isEmpty()) {
+					int closer_idx = closers.indexOf(c);// get the index of the
+														// closer, it matches
+														// the opener's index
+					// if the top value in the stack doesn't match closer's
+					// counterpart
+					// return false can't be a match
+					if (stack.peek() != openers.get(closer_idx)) {
+						return false;
+					} else {
+						// otherwise we pop off the stack cause it was a match
+						stack.pop();
+					}
+
+				} else {
+					// if we have a closer but the stack is already empty
+					return false;
+				}
+			}
+		}
+		return stack.isEmpty();
+	}
+
+	public static void main(String[] args) {
+		// my weird way of doing test cases
+		HashMap<String, Boolean> map = new HashMap<String, Boolean>();
+		map.put("{{{{{{{{{adfkjaefia}}}}}}}", false);
+		map.put("{{{{{{{{{[[[[[[kadfa{{{{{{{((({daljfdaf({{{[]}}kaldjfs})})))}}}}}}}]]]]]]}kjfela}}}}}}}}", true);
+		map.put("{{{[}}}}dafda", false);
+		map.put("{{{{{{{{{}}}}}}}}}", true);
+		map.put("[[[[[[[[[kafjalfeianfailfeja;fjai;efa;sfj]]]]]]]]]kjajdain", true);
+		map.put("", true);
+		map.put("((((((fjdalfeja((((alefjalisj(())))))))))))d", true);
+		map.put(")))(((d", false);
+		map.put("({)}", false);
+		for (String s : map.keySet()) {
+			System.out.println("Expected output for:" + s + " is " + map.get(s));
+			System.out.println("Actual output is: " + checkClosers(s) + "\n");
+		}
+	}
+}

challenge_2/java/jdfurlan/README.md

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+# SingleNumber
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+Maps are usually good for storing unique values since there 
+can only be 1 of any given key in a map. A set is also good for this
+but since we needed to track the occurence of a key, I decided to use map
+and track occurance with the value
+
+### 2. How to compile and run this code
+
+```
+javac SingleNumber.java
+java SingleNumber
+```
+
+### 3. How this program works
+
+If a key doesn't already exist in the map, put it in and set its value to 1
+Otherwise, key the value with associated key and increment by 1.
+Next, interate through the keySet and check when the value == 1, then return and exit

challenge_2/java/jdfurlan/src/SingleNumber.java

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+
+import java.util.HashMap;
+
+public class SingleNumber {
+
+	public static void main(String[] args) {
+		String[] arr = { "2", "a", "l", "3", "l", "4", "k", "2", "3", "4", "a", "6", "c", "4", "m", "6", "m", "k", "9",
+				"10", "9", "8", "7", "8", "10", "7" };
+		HashMap<String, Integer> map = new HashMap<String, Integer>();
+		for (String s : arr) {
+			if (map.containsKey(s)) {
+				map.put(s, map.get(s) + 1);
+			} else {
+				map.put(s, 1);
+			}
+		}
+		for (String s : map.keySet()) {
+			if (map.get(s) == 1) {
+				System.out.println(s);
+				System.exit(0);
+			}
+		}
+	}
+
+}

challenge_3/java/jdfurlan/README.md

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+# MajorityElement
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+I used a map to make sure all equivalent elements were mapped to the same key
+and tracked their frequencies with their value
+
+### 2. How to compile and run this code
+
+Compile the Node class first
+
+```
+javac MajorityElement.java
+java MajorityElement
+```
+
+### 3. How this program works
+
+As we iterate through the list, if the map has never seen this element before,
+place it in the map and initialize its frequency at 1. If we have seen it before, increment 
+its frequency, check if > n.length/2  and print if true, else update the frequency.

challenge_3/java/jdfurlan/src/MajorityElement.java

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+
+import java.util.HashMap;
+
+public class MajorityElement {
+
+	public static void main(String[] args) {
+		int[] nums = { 2, 2, 3, 7, 5, 7, 7, 7, 4, 7, 2, 7, 4, 5, 6, 7, 7, 8, 6, 7, 7, 8, 10, 12, 29, 30, 19, 10, 7, 7,
+				7, 7, 7, 7, 7, 7, 7 };
+		HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
+		for (int n : nums) {
+			if (map.containsKey(n)) {
+				int f = map.get(n) + 1;
+				if (f > (nums.length / 2)) {
+					System.out.println(n);
+					System.exit(0);
+				}
+				map.put(n, f);
+			} else {
+				map.put(n, 1);
+			}
+		}
+
+	}
+
+}

challenge_4/java/jdfurlan/README.md

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+# InvertBinaryTree
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+Trees suck if you haven't messed with them in a while!
+Swapping the values is relatively easy using an A-B-C swap approach.
+The approach to print was to do a [Breadth-First Traversal](https://www.cs.bu.edu/teaching/c/tree/breadth-first/)
+
+### 2. How to compile and run this code
+
+
+```
+javac InvertBinaryTree.java
+java InvertBinaryTree
+```
+
+### 3. How this program works
+
+Hand the root node to the swap function. There, while the node is not null
+you must take either child and store in a temp, then swap into the other child's place
+then swap the child you didn't store into a temp with the temp child.
+Call the swap function recursively with either child first, then the other child.
+BFT through the tree to print values by level.

challenge_4/java/jdfurlan/src/InvertBinaryTree.java

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+
+import java.util.LinkedList;
+import java.util.Queue;
+
+public class InvertBinaryTree {
+
+	public static void main(String[] args) {
+		Node root = new Node(4);
+		Node two = new Node(2);
+		Node seven = new Node(7);
+		Node three = new Node(3);
+		Node six = new Node(6);
+		Node nine = new Node(9);
+		Node one = new Node(1);
+
+		root.left = two;
+		root.right = seven;
+		two.left = one;
+		two.right = three;
+		seven.left = six;
+		seven.right = nine;
+
+		print(root);
+		invertBT(root);
+		System.out.println();
+		print(root);
+
+
+	}
+	//breadth-first traversal using Queue interface
+	private static void print(Node root) {
+		Queue<Node> children = new LinkedList<Node>();
+		children.add(root);
+		while (!children.isEmpty()) {
+			Node temp = children.poll();
+			System.out.print(temp.data);
+			if (temp.left != null)
+				children.add(temp.left);
+			if (temp.right != null)
+				children.add(temp.right);
+		}
+	}
+	//basic swap. Once swapped, swap left subtree then right subtree until null
+	private static void invertBT(Node root) {
+		if (root == null)
+			return;
+		Node temp = root.right;
+		root.right = root.left;
+		root.left = temp;
+		invertBT(root.left);
+		invertBT(root.right);
+	}
+
+}

challenge_4/java/jdfurlan/src/Node.java

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+
+public class Node {
+	int data;
+	Node left;
+	Node right;
+	public Node(int data){
+		this.data = data;
+		left = null;
+		right = null;
+	}
+}

challenge_5/java/jdfurlan/README.md

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+# Ranges
+###### Java 8
+
+### 1. Approach to Solving the problem
+
+I thought about various ways to compare values in a string.
+Using a boolean array is a common solution for comparing characters.
+
+### 2. How to compile and run this code
+
+
+```
+javac Ranges.java
+java Ranges
+```
+
+### 3. How this program works
+
+Create a boolean array of size 128, the ASCII code limit
+(this program wouldn't work for Unicode, too many chars)
+Every char value maps to an index in the array, walk through string
+s and put each boolean at the char index to true. Then walk through string
+t and as soon as you reach a boolean that is false, we know that's our missing char!