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/**
 * use inorder to determine the current node belongs to left or right.
 * use class variable to memorize the current position in preorder.
 */
class Solution {
    Map<Integer, Integer> loc;
    int p;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        loc = new HashMap<>(preorder.length);
        List<Integer> preorderList = new ArrayList<>(preorder.length);
        for (int i = 0; i < inorder.length; i++) {
            loc.put(inorder[i], i);
        }
        p = 0;
        return build(preorder, 0, preorder.length);
    }
    
    private TreeNode build(int[] pre, int l, int r) {
        if (r <= l) {
            return null;
        }
        TreeNode root = new TreeNode(pre[p++]);
        int idx = loc.get(root.val);
        root.left = build(pre, l, idx);
        root.right = build(pre, idx+1, r);
        return root;
    }
}