src/main/java/com/stevesun/solutions/_105.java

package com.stevesun.solutions;

import com.stevesun.common.classes.TreeNode;

import java.util.HashMap;
import java.util.Map;

/**Given preorder and inorder traversal of a tree, construct the binary tree.

 Note:
 You may assume that duplicates do not exist in the tree.*/
public class _105 {

	/**credit: https://discuss.leetcode.com/topic/29838/5ms-java-clean-solution-with-caching
	use HashMap as the cache so that accessing inorder index becomes O(1) time

	 Note: The first element of preorder array is the root!*/
	public TreeNode buildTree(int[] preorder, int[] inorder) {
		Map<Integer, Integer> inorderMap = new HashMap();
		for (int i = 0; i < inorder.length; i++) {
			inorderMap.put(inorder[i], i);
		}

		/**At the beginning, both start from 0 to nums.length-1*/
		return buildTree(preorder, 0, preorder.length-1, 0, inorder.length-1, inorderMap);
	}

	private TreeNode buildTree(int[] preorder, int preStart, int preEnd, int inStart, int inEnd, Map<Integer, Integer> inorderMap) {
		if (preStart > preEnd || inStart > inEnd) return null;

		TreeNode root = new TreeNode(preorder[preStart]);
		int inRoot = inorderMap.get(preorder[preStart]);
		int numsLeft = inRoot - inStart;

		/**It's easy to understand and remember:
		 * for the indices of inorder array:
		 * root.left should be inStart and inRoot-1 as new start and end indices
		 * root.right should be inRoot+1 and inEnd as new start and end indices
		 *
		 * since inRoot is being used already in this recursion call, that's why we use inRoot-1 and inRoot+1
		 * this part is the same for both Leetcode 105 and Leetcode 106.*/
		root.left = buildTree(preorder, preStart+1, preStart+numsLeft, inStart, inRoot-1, inorderMap);
		root.right = buildTree(preorder, preStart+numsLeft+1, preEnd, inRoot+1, inEnd, inorderMap);
		return root;
	}

}