coding-interview-patterns/java/Trees/BuildBinaryTree.java at main · iamthatdev/coding-interview-patterns · GitHub

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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;

}

coding-interview-patterns/java/Trees/BuildBinaryTree.java at main · iamthatdev/coding-interview-patterns · GitHub

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