Growing a Binary Tree

so i want to scan a string of an expression (ex: "1+2*3"), then put each of the element into a binary tree. So the operator will be the node, and the numbers will be the leaf. And below is my code:

Code:

double GrowTree(const string& expression)
{
	char number[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
	char operation[] = {'+', '-', '*', '/'};
	string tempEx = expression;
	TreeNode<char>* treeRoot = new TreeNode<char>;
	TreeNode<char>* treeCurrent = new TreeNode<char>;
	treeRoot = NULL;
	treeCurrent = treeRoot;

	while(!tempEx.empty())
	{
		for(int i=0; i<sizeof(number)/sizeof(char); i++)
		{
			if(tempEx[0] == number[i])
			{
				TreeNode<char>* newLeaf = new TreeNode<char>;
				newLeaf->Value = tempEx[0];

				if ( treeCurrent == NULL )
				{
					treeRoot = treeCurrent = newLeaf;
				}else
				{
					treeCurrent->Right = newLeaf;
				}
			}
		}


			//if find any operator sign
			if( (tempEx[0] == '+' || tempEx[0]=='-'))
			{
				if(treeCurrent->Right == NULL)
				{
					TreeNode<char>* newRoot = new TreeNode<char>;
					newRoot->Value = tempEx[0];
				
					//the operator will become the root
					newRoot->Left = treeRoot;
					treeCurrent = newRoot;

					//treeRoot->Right = treeCurrent;
				}else
				{
					TreeNode<char>* newRoot = new TreeNode<char>;
					newRoot->Value = tempEx[0];
					newRoot->Left = treeCurrent->Right;
					treeCurrent->Right = newRoot;
					treeCurrent = newRoot;
					//treeRoot->Right = treeCurrent;
				}
			}else
			{
				if ( (tempEx[0] == '*' || tempEx[0] == '/' ))
				{
					if(treeCurrent->Right == NULL)
					{
						TreeNode<char>* newChild = new TreeNode<char>;
						newChild->Value = tempEx[0];					
						//the operator will become the root
						newChild->Left = treeRoot;
						treeCurrent = newChild;
						//treeRoot->Right = treeCurrent;
					}else
					{
						TreeNode<char>* newChild = new TreeNode<char>;
						newChild->Value = tempEx[0];
						newChild->Left = treeCurrent->Right;
						treeCurrent->Right = newChild;
						treeCurrent = newChild;
						//treeRoot->Right = treeCurrent;
					}
				}
			}
		
		tempEx.erase(tempEx.begin());
	
	}


	return ValueOf(treeRoot);        //This is where the function calculate the expression, which is another function
}

At the end, i had no idea how to reverse and attach everything back to the 'treeRoot'. Can someone help me? Thank you very much

I could have reduce the code a little bit at checking the operators, but you know . I'll reduce it later when i the whole thing works

You need to extract the bulk of that into a seperate resursive function that returns the operator node each time, and the previous call of the function on the call stack takes the return value and attaches that as it's own left or right sub-tree as appropriate. The last recursive call left on the stack then returns the head of the tree.