Leetcode #144 Binary Tree Preorder Traversal

题目链接：https://leetcode.com/problems/binary-tree-preorder-traversal/

 

(非递归实现)二叉树的先序遍历。

class Solution
{
public:
    vector<int> preorderTraversal(TreeNode *root)
    {
        vector<int> output;
        if(root == NULL)
        {
            return output;
        }

        stack<TreeNode*> nodeStack;
        nodeStack.push(root);

        while(!nodeStack.empty())
        {
            TreeNode *topNode = nodeStack.top();
            TreeNode *topleftNode = topNode->left, *toprightNode = topNode->right;
            output.push_back(topNode->val);
            nodeStack.pop(); //先pop再push。
            
            if(toprightNode != NULL) //因为最后访问右边的节点，所以先把右边的节点压入栈。
                nodeStack.push(toprightNode);
            if(topleftNode != NULL)
                nodeStack.push(topleftNode);            
        }

        return output;
    }
};