通过前序遍历和中序遍历构造二叉树

class Solution {
    public int i = 0;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder == null || inorder == null){
            return null;
        }
        return buildTreeChild(preorder,inorder,0,inorder.length-1);
    }
    public TreeNode buildTreeChild(int[] preorder,int[] inorder,int inbegin,int inend){
        if(inbegin > inend){
            return null;
        }
        TreeNode root = new TreeNode(preorder[i]);
        int rootIndex = findIndex(inorder,inbegin,inend,preorder[i]);
        i++;
        root.left = buildTreeChild(preorder,inorder,inbegin,rootIndex - 1);
        root.right = buildTreeChild(preorder,inorder,rootIndex + 1,inend);
        return root;
    }
    public int findIndex (int[] inorder,int inbegin,int inend,int key){
        for(int i = inbegin;i<= inend;i++){
            if(inorder[i] == key){
                return i;
            }
        }
        return -1;
    }
}