力扣-105-从前序与中序遍历序列构造二叉树

public TreeNode buildTree(int[] preorder, int[] inorder) {
    if(preorder == null || inorder == null 
    || preorder.length <= 0 || inorder.length <= 0 
    || preorder.length != inorder.length)   return null;

    return construct(0,preorder.length - 1,preorder,0,inorder.length-1,inorder);
}

private TreeNode construct(int prel,int prer,int[] pre,int ml,int mr,int[] mid){
    TreeNode root = new TreeNode(pre[prel]);
    
    if(prel == prer && ml == mr)    return root;
    int index = ml;
    while(root.val != mid[index] && index <= mr){
        index ++;//中序中根结点的位置
    }
    int leftcount = index - ml;
    if(leftcount > 0){
        root.left = construct(prel+1,prel+leftcount,pre,ml,index-1,mid);
    }
    if(leftcount < mr - ml){
        root.right = construct(prel+leftcount+1,prer,pre,index+1,mr,mid);
    }

    return root;
}