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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

  用stack，每次先push right再push left Time/Space O(n)   实现：

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        
        vector<int> res;
        if(!root) return res;
        
        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty()){
            TreeNode* cur = st.top();
            st.pop();
            res.push_back(cur->val);
            if(cur->right) st.push(cur->right);
            if(cur->left) st.push(cur->left);
        }
        
        return res;
        
    }
};