LeetCode Binary Tree Preorder Traversal 先根遍历

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
if(!root)  return vector<int>();

vector<int> ans;
stack<TreeNode *>  stac;
stac.push(root);

while( !stac.empty() )
{
TreeNode *t=stac.top();
ans.push_back(t->val);
stac.pop();//只需要孩子都压栈，父亲无用
if(t->right)    stac.push(t->right);
if(t->left)     stac.push(t->left);
}
return ans;
}
};