Java Chapter 11: Trees

java/Trees/BalancedBinaryTreeValidation.java

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+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BalancedBinaryTreeValidation {
+    public boolean balancedBinaryTreeValidation(TreeNode root) {
+        return getHeightImbalance(root) != -1;
+    }
+
+    private int getHeightImbalance(TreeNode node) {
+        // Base case: if the node is null, its height is 0.
+        if (node == null) {
+            return 0;
+        }
+        // Recursively get the height of the left and right subtrees. If
+        // either subtree is imbalanced, propagate -1 up the tree.
+        int leftHeight = getHeightImbalance(node.left);
+        int rightHeight = getHeightImbalance(node.right);
+        if (leftHeight == -1 || rightHeight == -1) {
+            return -1;
+        }
+        // If the current node's subtree is imbalanced
+        // (height difference > 1), return -1.
+        if (Math.abs(leftHeight - rightHeight) > 1) {
+            return -1;
+        }
+        // Return the height of the current subtree.
+        return 1 + Math.max(leftHeight, rightHeight);
+    }
+}

java/Trees/BinarySearchTreeValidation.java

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+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BinarySearchTreeValidation {
+    public boolean binarySearchTreeValidation(TreeNode root) {
+        // Start validation at the root node. The root node can contain any 
+        // value, so set the initial lower and upper bounds to null.
+        return isWithinBounds(root, null, null);
+    }
+
+    private boolean isWithinBounds(TreeNode node, Integer lowerBound, Integer upperBound) {
+        // Base case: if the node is null, it satisfies the BST condition.
+        if (node == null) {
+            return true;
+        }
+        // If the current node's value is not within the valid bounds, this 
+        // tree is not a valid BST.
+        if (lowerBound != null && node.val <= lowerBound || upperBound != null && upperBound <= node.val) {
+            return false;
+        }
+        // If the left subtree isn't a BST, this tree isn't a BST.
+        if (!isWithinBounds(node.left, lowerBound, node.val)) {
+            return false;
+        }
+        // Otherwise, return true if the right subtree is also a BST.
+        return isWithinBounds(node.right, node.val, upperBound);
+    }
+}

java/Trees/BinaryTreeColumns.java

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+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BinaryTreeColumns {
+    public class Pair {
+        TreeNode node;
+        int column;
+        public Pair(TreeNode node, int column) {
+            this.node = node;
+            this.column = column;
+        }
+    }
+
+    public List<List<Integer>> binaryTreeColumns(TreeNode root) {
+        if (root == null) {
+            return new ArrayList<>();
+        }
+        Map<Integer, List<Integer>> columnMap = new HashMap<>();
+        int leftmostColumn, rightmostColumn;
+        leftmostColumn = rightmostColumn = 0;
+        Queue<Pair> queue = new ArrayDeque<>();
+        queue.offer(new Pair(root, 0));
+        while (!queue.isEmpty()) {
+            Pair pair = queue.poll();
+            TreeNode node = pair.node;
+            int column = pair.column;
+            if (node != null) {
+                // Add the current node's value to its corresponding list in the hash 
+                // map.
+                List<Integer> columnList = columnMap.getOrDefault(column, new ArrayList<>());
+                columnList.add(node.val);
+                columnMap.put(column, columnList);
+                leftmostColumn = Math.min(leftmostColumn, column);
+                rightmostColumn = Math.max(rightmostColumn, column);
+                // Add the current node's children to the queue with their respective
+                // column ids.
+                queue.offer(new Pair(node.left, column - 1));
+                queue.offer(new Pair(node.right, column + 1));
+            }
+        }
+        // Construct the output list by collecting values from each column in the hash   
+        // map in the correct order.
+        List<List<Integer>> res = new ArrayList<>();
+        for (int i = leftmostColumn; i <= rightmostColumn; i++) {
+            List<Integer> column = columnMap.get(i);
+            res.add(column);
+        }
+        return res;
+    }
+}

java/Trees/BinaryTreeSymmetry.java

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+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BinaryTreeSymmetry {
+    public boolean binaryTreeSymmetry(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        return compareTrees(root.left, root.right);
+    }
+
+    private boolean compareTrees(TreeNode node1, TreeNode node2) {
+        // Base case: if both nodes are null, they're symmetric.
+        if (node1 == null && node2 == null) {
+            return true;
+        }
+        // If one node is null and the other isn't, they aren't symmetric.
+        if (node1 == null || node2 == null) {
+            return false;
+        }
+        // If the values of the current nodes don't match, trees aren't symmetric.
+        if (node1.val != node2.val) {
+            return false;
+        }
+        // Compare the 'node1's left subtree with 'node2's right subtree. If these 
+        // aren't symmetric, the whole tree is not symmetric.
+        if (!compareTrees(node1.left, node2.right)) {
+            return false;
+        }
+        // Compare the 'node1's right subtree with 'node2's left subtree. 
+        return compareTrees(node1.right, node2.left);
+    }
+}

java/Trees/BuildBinaryTree.java

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+import java.util.HashMap;
+import java.util.Map;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BuildBinaryTree {
+    int preorderIndex = 0;
+    Map<Integer, Integer> inorderIndexesMap = new HashMap<>();
+
+    public TreeNode buildBinaryTree(int[] preorder, int[] inorder) {
+        // Populate the hash map with the inorder values and their indexes.
+        for (int i = 0; i < inorder.length; i++) {
+            inorderIndexesMap.put(inorder[i], i);
+        }
+        // Build the tree and return its root node.
+        return buildSubtree(0, inorder.length - 1, preorder, inorder);
+    }
+
+    private TreeNode buildSubtree(int left, int right, int[] preorder, int[] inorder) {
+        // Base case: if no elements are in this range, return None.
+        if (left > right) {
+            return null;
+        }
+        int val = preorder[preorderIndex];
+        // Set 'inorderIndex' to the index of the same value pointed at by
+        // 'preorderIndex'.
+        int inorderIndex = inorderIndexesMap.get(val);
+        TreeNode node = new TreeNode(val);
+        // Advance 'preorderIndex' so it points to the value of the next
+        // node to be created.
+        preorderIndex++;
+        // Build the left and right subtrees and connect them to the current
+        // node.
+        node.left = buildSubtree(left, inorderIndex - 1, preorder, inorder);
+        node.right = buildSubtree(inorderIndex + 1, right, preorder, inorder);
+        return node;
+    }
+}

java/Trees/InvertBinaryTreeIterative.java

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+import java.util.Stack;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class InvertBinaryTreeIterative {
+    public TreeNode invertBinaryTreeIterative(TreeNode root) {
+        if (root == null) {
+            return null;
+        }
+        Stack<TreeNode> stack = new Stack<>();
+        stack.push(root);
+        while (!stack.isEmpty()) {
+            TreeNode node = stack.pop();
+            // Swap the left and right subtrees of the current node.
+            TreeNode tmp = node.left;
+            node.left = node.right;
+            node.right = tmp;
+            // Push the left and right subtrees onto the stack.
+            if (node.left != null) {
+                stack.push(node.left);
+            }
+            if (node.right != null) {
+                stack.push(node.right);
+            }
+        }
+        return root;
+    }
+}

java/Trees/InvertBinaryTreeRecursive.java

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+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class InvertBinaryTreeRecursive {
+    public TreeNode invertBinaryTreeRecursive(TreeNode root) {
+        // Base case: If the node is null, there's nothing to invert.
+        if (root == null) {
+            return root;
+        }
+        // Swap the left and right subtrees of the current node.
+        TreeNode tmp = root.left;
+        root.left = root.right;
+        root.right = tmp;
+        // Recursively invert the left and right subtrees.
+        invertBinaryTreeRecursive(root.left);
+        invertBinaryTreeRecursive(root.right);
+        return root;
+    }
+}

java/Trees/KthSmallestNumberInBSTIterative.java

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+import java.util.Stack;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class KthSmallestNumberInBSTIterative {
+    public int kthSmallestNumberInBSTIterative(TreeNode root, int k) {
+        Stack<TreeNode> stack = new Stack<>();
+        TreeNode node = root;
+        while (!stack.isEmpty() || node != null) {
+            // Move to the leftmost node and add nodes to the stack as we go so they 
+            // can be processed in future iterations.
+            while (node != null) {
+                stack.push(node);
+                node = node.left;
+            }
+            // Pop the top node from the stack to process it, and decrement 'k'.
+            node = stack.pop();
+            k--;
+            // If we have processed 'k' nodes, return the value of the 'k'th smallest 
+            // node.
+            if (k == 0) {
+                return node.val;
+            }
+            // Move to the right subtree.
+            node = node.right;
+        }
+        return -1;
+    }
+}

java/Trees/KthSmallestNumberInBSTRecursive.java

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+import java.util.ArrayList;
+import java.util.List;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class KthSmallestNumberInBSTRecursive {
+    public int kthSmallestNumberInBSTRecursive(TreeNode root, int k) {
+        List<Integer> sortedList = new ArrayList<>();
+        inorder(root, sortedList);
+        return sortedList.get(k - 1);
+    }
+
+    // Inorder traversal function to attain a sorted list of nodes from the BST.
+    private void inorder(TreeNode node, List<Integer> sortedList) {
+        if (node == null) {
+            return;
+        }
+        inorder(node.left, sortedList);
+        sortedList.add(node.val);
+        inorder(node.right, sortedList);
+    }
+}