LeetCode_144_二叉树的前序遍历

题目描述：

给定一个二叉树，返回它的 前序 遍历。

 示例:

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

输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

递归的写法没什么可说的，基本功

/**
 * 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:
    void preorder(TreeNode* root,vector<int> &res){
        if(root==NULL)
            return ;
        res.push_back(root->val);
        preorder(root->left,res);
        preorder(root->right,res);
    }
    
    vector<int> preorderTraversal(TreeNode* root) {
        if(root==NULL)
            return {};
        vector<int> res;
        preorder(root,res);
        return res;
    }
};

迭代算法：使用stack来实现迭代算法，完成前序遍历，右子树先入栈，然后再左子树入栈

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