剑指offer第5题：重建二叉树

    HashMap<Integer,Integer> map = new HashMap<>();
    int preIndex = 0;//遍历前序遍历的索引值
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        for(int i = 0 ; i < inorder.length ; i++){//首先抽象出中序遍历
            map.put(inorder[i] , i);
        }
        return help(preorder , inorder , 0 , preorder.length - 1);
    }

    private TreeNode help(int[] preorder , int[] inorder , int start , int end){
        if(start > end) return null;
        int val = preorder[preIndex++];//获取头结点
        TreeNode node = new TreeNode(val);
        int midIndex = map.get(val);
        node.left = help(preorder , inorder , start , midIndex - 1);//构建左节点
        node.right = help(preorder , inorder , midIndex+1 , end);//构建右节点
        return node;
    }