【题目描述】Given a binary tree, return the preorder traversal of its nodes’ values.For example:Given binary tree{1,#,2,3},
1

2
/
3
return[1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?
非递归法实现二叉树的先序遍历

【解题思路】用迭代法做深度优先搜索的技巧就是使用一个显式声明的Stack存储遍历到节点，替代递归中的进程栈，实际上空间复杂度还是一样的。对于先序遍历，我们pop出栈顶节点，记录它的值，然后将它的左右子节点push入栈，以此类推。

【考察内容】二叉树的先序遍历，栈

/**
 * Definition for binary tree
 * 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) {
        stack<TreeNode*> s;
        vector<int> res;
        if(root!=NULL) 
            s.push(root);
        while(!s.empty()){
            TreeNode *curr = s.top();
            s.pop();
            res.push_back(curr->val);
            if(curr->right!=NULL) s.push(curr->right);
            if(curr->left!=NULL) s.push(curr->left);
        }
        return res;
    }
};