剑指 Offer 07. 重建二叉树中等难度

class Solution {
    HashMap<Integer, Integer> dic = new HashMap<>();
    int[] po;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        po = preorder;
        for(int i = 0; i < inorder.length; i++) 
            dic.put(inorder[i], i);
        return recur(0, 0, inorder.length - 1);
    }
    TreeNode recur(int pre_root, int in_left, int in_right) {
        if(in_left > in_right) return null;
        TreeNode root = new TreeNode(po[pre_root]);
        int i = dic.get(po[pre_root]);
        root.left = recur(pre_root + 1, in_left, i - 1);
        root.right = recur(pre_root + i - in_left + 1, i + 1, in_right);
        return root;
    }
}