已知前序和中序

public class TreeNode
   {
   public int val;
   public TreeNode left;
   public TreeNode right;
   public TreeNode(int x) { val = x; }
}
   public TreeNode BuildTree1(int[] preorder, int[] inorder)
   {
       if (preorder == null || inorder == null) return null;
       return ReBuildTree1(preorder,0,preorder.Length-1,inorder,0, inorder.Length-1);
   }
   TreeNode ReBuildTree1(int[] preTree,int startPre,int endPre,int[] inTree,int startIn,int endIn)
   {
       if (startPre > endPre || startIn > endIn) return null;
       //前序遍历数组的第一个元素就是子树的根节点
       TreeNode root = new TreeNode(preTree[startPre]);
       //在中序中找到和根节点相同的节点
       for (int i = startIn; i <= endIn; i++)
       {
           if(inTree[i] == preTree[startPre])
           {
               //其中(i-startIn)为中序排序中左子树节点的个数
               int leftLength = i - startIn;//左子树的长度
               int preStartRight = startPre + leftLength + 1;//右子树开始的
               int preStartLeft = startPre + 1;//左子树开始
               //左子树
               root.left = ReBuildTree1(preTree, preStartLeft, startPre + leftLength, inTree, startIn, i - 1);
               root.right = ReBuildTree1(preTree, preStartRight, endPre, inTree, i+1, endIn);
           }
       }
       return root;
   }

已知后序和中序

 public TreeNode BuildTree(int[] inorder, int[] postorder)
    {
        if (inorder == null || postorder == null) return null;
        return ReBuildTree(inorder, 0, inorder.Length - 1, postorder, 0, postorder.Length - 1);
    }
    TreeNode ReBuildTree(int[] inorder, int startIn, int endIn, int[] postorder, int startPost, int endPost)
    {
        if (startIn > endIn || startPost < 0) return null;
        //后序序遍历数组的最后一个元素就是子树的根节点
        TreeNode root = new TreeNode(postorder[endPost]);
        //在中序中找到和根节点相同的节点
        for (int i = startIn; i <= endIn; i++)
        {
            if (inorder[i] == postorder[endPost])
            {
                //i-startIn为左子树的长度
                int postStartLeft = startPost+i-startIn;
                int postStartRight = startPost + i - startIn-1;
                //左子树
                root.left = ReBuildTree(inorder, startIn , i-1, postorder, startPost, postStartRight);
                root.right = ReBuildTree(inorder, i + 1,endIn , postorder, postStartLeft, endPost-1);
            }
        }
        return root;
    }