[leetcode]binary-tree-preorder-Traversal

題目描述：

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

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

實作思路：

要實作非遞歸的二叉樹前序周遊，可以借助棧來實作，前序周遊是根-左-右，先把根壓棧然後彈出到vector中，再依次将右孩子壓棧、左孩子壓棧，然後疊代進行，每次都彈出棧底的結點~

具體實作：

/**
 * 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) {
        vector<int> res;
        if(!root){
            return res;
        }
        stack<TreeNode *> st;
        st.push(root);
        while(st.size()){
            TreeNode * temp;
            temp = st.top();
            res.push_back(temp->val);
            st.pop();
            if(temp->right){
                st.push(temp->right);
            }
            if(temp->left){
                st.push(temp->left);
            }
        }
        return res;
    }
};