leetcode(106)从中序遍历和后序遍历构建二叉树

class Solution {
    public HashMap<Integer,Integer> map = new HashMap<>();
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        int len = postorder.length;
        for(int i=0;i<len;++i) {
            map.put(inorder[i], i);
        }
        TreeNode root = buildTree2(postorder, inorder, 0, len-1, len-1);
        return root;
    }
    public TreeNode buildTree2(int[] preorder, int[] inorder, int start, int end, int last) {
        if(start>end) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[last]);
        int middle = map.get(preorder[last]);
        root.left = buildTree2(preorder,inorder,start,middle-1,last-1-(end-middle));
        root.right = buildTree2(preorder,inorder,middle+1,end,last-1);
        return root;
    }
}