leetcode 144. Binary Tree Preorder Traversal 二叉树前序遍历 + 深度优先遍历DFS-蒲公英云

leetcode 144. Binary Tree Preorder Traversal 二叉树前序遍历 + 深度优先遍历DFS

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

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].

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

本题考察的是二叉树的前序遍历的非递归实现。

嗯嗯，直接看代码吧！

代码如下：

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
/*class TreeNode 
{
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x) { val = x; }
}*/
public class Solution
{
    List<Integer> res=new ArrayList<>();
    public List<Integer>  preorderTraversal(TreeNode root) 
    {
        if(root==null)
            return res;
        else
        {
            //preOrder(root);
            byIteratorPreOrder(root);
            return res;
        }
    }
    //前序遍历非递归
    void byIteratorPreOrder(TreeNode root) 
    {
        Stack<TreeNode> myStack=new Stack<>();
        myStack.push(root);
        //就是栈顶元素top出栈，然后访问top，然后添加top的子节点
        while(myStack.isEmpty()==false)
        {
            TreeNode top=myStack.peek();
            myStack.pop();
            res.add(top.val);
            //要先添加右结点，然后是左结点
            if(top.right!=null)
                myStack.push(top.right);
            if(top.left!=null)
                myStack.push(top.left);
        }
    }
    //递归前序遍历
    void preOrder(TreeNode root) 
    {
        if(root==null)
            return;
        else
        {
            res.add(root.val);
            preOrder(root.left);
            preOrder(root.right);
        }
    }
}

下面是C++的做法，就是使用DFS递归遍历来做

代码如下：

#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <string>
#include <climits>
#include <algorithm>
#include <sstream>
#include <functional>
#include <bitset>
#include <numeric>
#include <cmath>
#include <regex>
#include <iomanip>
#include <cstdlib>
#include <ctime>
using namespace std;
/*
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;
        stack<TreeNode*> skt;
        while (true)
        {
            while (root != NULL)
            {
                res.push_back(root->val);
                skt.push(root);
                root = root->left;
            }
            if (skt.empty())
                break;
            else
            {
                root = skt.top()->right;
                skt.pop();
            }
        }
        return res;
    }
};