144.binary-tree-preorder-traversal 二叉树的前序遍历

前序遍历即中左右，前中后序遍历区别就在于中节点是在前、中还是后。
利用栈实现二叉树的迭代遍历：

#include <stack>
#include <vector>
using std::stack;
using std::vector;
class Solution {
  public:
    vector<int> preorderTraversal(TreeNode *root) {
        stack<TreeNode *> stk;
        vector<int> res;
        if (root == nullptr)
            return res;
        stk.push(root);
        while (!stk.empty()) {
            TreeNode *node = stk.top();
            stk.pop();
            res.push_back(node->val);
            if (node->right != nullptr)
                stk.push(node->right);
            if (node->left != nullptr)
                stk.push(node->left);
        }
        return res;
    }
};

递归法：

class Solution {
public:
    vector<int> res;
    vector<int> preorderTraversal(TreeNode* root) {
        if (root == nullptr)
            return res;
        res.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);
        return res;
    }
};