LeetCode:Binary Tree Preorder Traversal

问题描述：

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

思路：借助栈来实现树的先序遍历。

代码：

vector<int> preorderTraversal(TreeNode * root)
{
    vector<int> preorderSequence;
    stack<TreeNode * > treeNodeStack;
    TreeNode * node = root;
    if(root == NULL)
        return preorderSequence;
    preorderSequence.push_back(node->val);
    if(node->right)
        treeNodeStack.push(node->right);
    if(node->left)
        treeNodeStack.push(node->left);
    while(!treeNodeStack.empty())
    {
        node = treeNodeStack.top();
        treeNodeStack.pop();
        preorderSequence.push_back(node->val);
        if(node->right)
            treeNodeStack.push(node->right);
        if(node->left)
            treeNodeStack.push(node->left);
    }
    return preorderSequence;
}