【剑指Offer】重建二叉树

通过中序数组来切分左树和右树的子序列，在中序数组中找到和根节点相同的元素。后续的关键是找准左树右树在中序和先序数组中的子序列的位置。

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder == null || inorder == null){
            return null;
        }
        return helper(preorder,inorder,0,preorder.length-1,0,inorder.length-1);
    }
    public TreeNode helper(int[] preorder ,int[] inorder,int preStart, int preEnd, int inStart, int inEnd){
        if(preStart > preEnd || inStart > inEnd){
            return null;   
        }
        TreeNode root = new TreeNode(preorder[preStart]);
        for(int i = inStart; i <= inEnd; i++){
            if(inorder[i] == preorder[preStart]){
                root.left = helper(preorder, inorder,preStart+1,preStart+i-inStart,inStart,i-1);
                root.right = helper(preorder, inorder,preStart+i-inStart+1,preEnd,i+1,inEnd);
            }
        }
        return root;
    }
}