Construct Binary Tree from Preorder and Inorder Traversal

Java

public class Solution {
		//HashMap to locate current root index to split the tree in the inorder array
		HashMap<Integer, Integer> idxMap = new HashMap<>(); 
		int[] preorder;
		int[] inorder;
		int preIdx = 0;
		
		public TreeNode buildTree(int[] preorder, int[] inorder) {
			this.preorder = preorder;
			this.inorder = inorder; 
		//corner case 
			if (preorder.length != inorder.length) return null;
			
			int idx = 0;
			for (int val : inorder) {
				idxMap.put(val, idx++);                  //HashMap建立val和idx的套路
			}
			
			return helper(0, preorder.length);  //这里为什么不用 preorder.length - 1 呢？
		}
		
		private TreeNode helper(int start, int end) {
			if (start == end) return null;
		
		int curRootVal = preorder[preIdx];
			TreeNode curRoot = new TreeNode(curRootVal);
		
		preIdx ++;
			int curRootIdx = idxMap.get(curRootVal);
				
			curRoot.left = helper(start, curRootIdx);
			curRoot.right = helper(curRootIdx + 1, end);
			return curRoot;
		}
}