LeetCode144二叉树前序遍历

题目链接

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/

题解一：递归

// Problem: LeetCode 144
// URL: https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/
// Tags: Tree Recursion Stack
// Difficulty: Medium

#include <iostream>
#include <vector>
using namespace std;

struct TreeNode{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x):val(x), left(nullptr), right(nullptr){}
};

class Solution{
public:
    void traversal(TreeNode* root, vector<int>& result){
        if (root != nullptr){
            result.push_back(root->val);
            if(root->left!=nullptr)
                traversal(root->left, result);
            if(root->right!=nullptr)
                traversal(root->right, result);
        }
    }
    vector<int> preorderTraversal(TreeNode* root){
        vector<int> result;
        traversal(root, result);
        return result;
    }
};

int main()
{
    cout << "helloworld" << endl;
    // system("pause");
    return 0;
}

题解二：非递归

• 需要用到栈
• 注意循环条件
• 注意是先把右子树存入栈中

// Problem: LeetCode 144
// URL: https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/
// Tags: Tree Recursion Stack
// Difficulty: Medium

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

struct TreeNode{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x):val(x), left(nullptr), right(nullptr){}
};

class Solution{
public:
    vector<int> preorderTraversal(TreeNode* root){
        vector<int> result;
        stack<TreeNode*> nodes;
        if(root!=nullptr)
            nodes.push(root);

        while(!nodes.empty()){
            root = nodes.top();
            nodes.pop();
            if(root!=nullptr){
                result.push_back(root->val);
                nodes.push(root->right);
                nodes.push(root->left);
            }
        }
        return result;
    }
};

int main()
{
    cout << "helloworld" << endl;
    // system("pause");
    return 0;
}

题解三：非递归（通过栈模拟递归）

思路：https://www.cnblogs.com/chouxianyu/p/13293284.html

// Problem: LeetCode 144
// URL: https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/
// Tags: Tree Recursion Stack
// Difficulty: Medium

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

struct TreeNode{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x):val(x), left(nullptr), right(nullptr){}
};

class Solution{
public:
    vector<int> preorderTraversal(TreeNode* root){
        vector<int> result;
        stack<TreeNode*> nodes;
        if(root!=nullptr)
            nodes.push(root);

        while(!nodes.empty()){
            // 取出节点
            root = nodes.top();
            nodes.pop();
            // 将节点对应的树拆解成前序，模拟递归解法中的函数调用
            if(root!=nullptr){
                if(root->right!=nullptr)
                    nodes.push(root->right);
                if(root->left!=nullptr)
                    nodes.push(root->left);
                nodes.push(root);
                nodes.push(nullptr);  // 设置nullptr标志
            }
            // nullptr代表栈顶对应的树（根、左子树、右子树）已按照指定顺序遍历，一层递归函数运行结束
            else{
                result.push_back(nodes.top()->val);
                nodes.pop();
            }
        }
        return result;
    }
};

int main()
{
    cout << "helloworld" << endl;
    // system("pause");
    return 0;
}

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作者：@臭咸鱼

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