96 lines
3.8 KiB
Java
96 lines
3.8 KiB
Java
package DataStructures.Trees;
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import java.util.HashMap;
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import java.util.Map;
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import DataStructures.Trees.BinaryTree.Node;
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/**
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* Approach: Naive Solution: Create root node from first value present in
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* preorder traversal. Look for the index of root node's value in inorder
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* traversal. That will tell total nodes present in left subtree and right
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* subtree. Based on that index create left and right subtree.
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* Complexity:
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* Time: O(n^2) for each node there is iteration to find index in inorder array
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* Space: Stack size = O(height) = O(lg(n))
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*
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* Optimized Solution: Instead of iterating over inorder array to find index of
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* root value, create a hashmap and find out the index of root value.
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* Complexity:
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* Time: O(n) hashmap reduced iteration to find index in inorder array
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* Space: O(n) space taken by hashmap
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*
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*/
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public class CreateBinaryTreeFromInorderPreorder {
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public static void main(String[] args) {
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test(new Integer[] {}, new Integer[] {}); // empty tree
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test(new Integer[] { 1 }, new Integer[] { 1 }); // single node tree
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test(new Integer[] { 1, 2, 3, 4 }, new Integer[] { 1, 2, 3, 4 }); // right skewed tree
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test(new Integer[] { 1, 2, 3, 4 }, new Integer[] { 4, 3, 2, 1 }); // left skewed tree
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test(new Integer[] { 3, 9, 20, 15, 7 }, new Integer[] { 9, 3, 15, 20, 7 }); // normal tree
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}
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private static void test(final Integer[] preorder, final Integer[] inorder) {
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System.out.println("\n====================================================");
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System.out.println("Naive Solution...");
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BinaryTree root = new BinaryTree(createTree(preorder, inorder, 0, 0, inorder.length));
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System.out.println("Preorder Traversal: ");
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root.preOrder(root.getRoot());
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System.out.println("\nInorder Traversal: ");
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root.inOrder(root.getRoot());
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System.out.println("\nPostOrder Traversal: ");
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root.postOrder(root.getRoot());
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Map<Integer, Integer> map = new HashMap<>();
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for (int i = 0; i < inorder.length; i++) {
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map.put(inorder[i], i);
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}
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BinaryTree optimizedRoot = new BinaryTree(createTreeOptimized(preorder, inorder, 0, 0, inorder.length, map));
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System.out.println("\n\nOptimized solution...");
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System.out.println("Preorder Traversal: ");
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optimizedRoot.preOrder(root.getRoot());
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System.out.println("\nInorder Traversal: ");
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optimizedRoot.inOrder(root.getRoot());
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System.out.println("\nPostOrder Traversal: ");
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optimizedRoot.postOrder(root.getRoot());
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}
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private static Node createTree(final Integer[] preorder, final Integer[] inorder,
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final int preStart, final int inStart, final int size) {
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if (size == 0) {
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return null;
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}
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Node root = new Node(preorder[preStart]);
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int i = inStart;
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while (preorder[preStart] != inorder[i]) {
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i++;
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}
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int leftNodesCount = i - inStart;
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int rightNodesCount = size - leftNodesCount - 1;
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root.left = createTree(preorder, inorder, preStart + 1, inStart, leftNodesCount);
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root.right = createTree(preorder, inorder, preStart + leftNodesCount + 1, i + 1,
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rightNodesCount);
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return root;
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}
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private static Node createTreeOptimized(final Integer[] preorder, final Integer[] inorder,
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final int preStart, final int inStart, final int size,
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final Map<Integer, Integer> inorderMap) {
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if (size == 0) {
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return null;
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}
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Node root = new Node(preorder[preStart]);
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int i = inorderMap.get(preorder[preStart]);
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int leftNodesCount = i - inStart;
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int rightNodesCount = size - leftNodesCount - 1;
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root.left = createTreeOptimized(preorder, inorder, preStart + 1, inStart,
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leftNodesCount, inorderMap);
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root.right = createTreeOptimized(preorder, inorder, preStart + leftNodesCount + 1,
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i + 1, rightNodesCount, inorderMap);
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return root;
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}
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}
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