二叉树的遍历算法

二叉树的先序、中序、后序以及层次遍历算法

非递归版本：

vector<int> postorderTraversal(TreeNode *root){
	vector<int> vect;
	if (!root)
	{
		return vect;
	}
	stack<TreeNode*> stackData;
	stackData.push(root);
	TreeNode* cur = NULL;
	TreeNode* pre = NULL;
	while (!stackData.empty())
	{
		cur=stackData.top();
		if (cur->left==NULL&&cur->right==NULL || (pre!=NULL)&&(pre==cur->left||pre==cur->right))
		{
			vect.push_back(cur->val);
			pre=cur;
			stackData.pop();
		}else{
			if (cur->right)
			{
				stackData.push(cur->right);
			}

			if (cur->left)
			{
				stackData.push(cur->left);
			}
		}
	}
	return vect;
}


vector<int> preordertTraversal(TreeNode *root){
	
	vector<int> vect;
	
	if (!root)
	{
		return vect;
	}
	stack<TreeNode*> stackData;
	TreeNode *pNode = root;
	while (pNode||!stackData.empty())
	{
		while (pNode)
		{
			vect.push_back(pNode->val);
			stackData.push(pNode);
			pNode=pNode->left;
		}

		if (!stackData.empty())
		{
			pNode=stackData.top();
			stackData.pop();
			pNode=pNode->right;
		}
	}

	return vect;
}

vector<int> inorderTraversal(TreeNode *root){
	vector<int> inorder;
	stack<TreeNode*> stackData;
	TreeNode *pNode=root;
	while(pNode||!stackData.empty()){
		while(pNode){
			stackData.push(pNode);
			pNode=pNode->left;
		}

		if(!stackData.empty()){
			pNode=stackData.top();
			inorder.push_back(pNode->val);
			stackData.pop();
			pNode = pNode->right;
		}

	}
	return inorder;
}

vector<vector<int> > levelOrderBottom(TreeNode *root){
	vector<vector<int> > result;
	if (!root)
	{
		return result;
	}
	vector<int> curlevel;
	queue<TreeNode*> q;
	q.push(root);
	q.push(NULL);
	while (!q.empty())
	{
		TreeNode * node = q.front();
		q.pop();

		if(node)
		{
			curlevel.push_back(node->val);
			if (node->left)
			{
				q.push(node->left);
			}
			if (node->right)
			{
				q.push(node->right);
			}
		}else {
			result.push_back(curlevel);
			curlevel.clear();
			if(!q.empty())  //该处相当关键 否则会出现死循环
				q.push(NULL);
		}
	}
	reverse(result.begin(),result.end());
	return result;  
}