［LeetCode］Binary Tree Preorder Traversal

解题思路：

前序遍历。

一，递归法(超简单)： 

/** * 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> ret;    vector<int> preorderTraversal(TreeNode* root) {        ret.clear();        DFS(root);        return ret;    }    void DFS(TreeNode* root){        if (root == NULL) return ;        ret.push_back(root->val);        DFS(root->left);        DFS(root->right);    }};

迭代法：

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