Binary Tree Preorder Traversal(C++二叉树的前序遍历)

/**
 * 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) {
        vector<int> a;
        deque <TreeNode*> c;  //定义一个空的队列
        c.push_back(root);
        while (!c.empty()) {  //如果队列不为空
            TreeNode* temp = c.front();  //返回队列的第一个元素
            if (temp) {  //如果是非空结点
                a.push_back(temp->val);
                c.pop_front();  //出队列

                c.push_front(temp->right); //右孩子
                c.push_front(temp->left);  //左孩子
            }
            else {
                c.pop_front();  //出队列
            }
        }
        return a;
    }
};