[LeetCode]144.Binary Tree Preorder Traversal

【主题】

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?

【代码一】

/*********************************
*   日期：2014-10-15
*   作者：SJF0115
*   题号: 144.Binary Tree Preorder Traversal
*   来源：https://oj.leetcode.com/problems/binary-tree-preorder-traversal/
*   结果：AC
*   来源：LeetCode
*   总结：
**********************************/
#include <iostream>
#include <malloc.h>
#include <vector>
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> v;
    void PreOrder(TreeNode *root){
        if (root == NULL){
            return;
        }
        v.push_back(root->val);
        PreOrder(root->left);
        PreOrder(root->right);
    }
    vector<int> preorderTraversal(TreeNode *root) {
        PreOrder(root);
        return v;
    }
};

//按先序序列创建二叉树
int CreateBTree(TreeNode* &T){
    char data;
    //按先序次序输入二叉树中结点的值（一个字符）。‘#’表示空树
    cin>>data;
    if(data == '#'){
        T = NULL;
    }
    else{
        T = (TreeNode*)malloc(sizeof(TreeNode));
        //生成根结点
        T->val = data-'0';
        //构造左子树
        CreateBTree(T->left);
        //构造右子树
        CreateBTree(T->right);
    }
    return 0;
}

int main() {
    Solution solution;
    TreeNode* root(0);
    CreateBTree(root);
    vector<int> v = solution.preorderTraversal(root);
    for(int i = 0;i < v.size();i++){
        cout<<v[i]<<endl;
    }
}

【代码二】

非递归实现

class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> v;
        stack<TreeNode*> s;
        TreeNode* p = root;
        //栈不空或者p不空时循环
        while(p != NULL || !s.empty()){
            if(p != NULL){
                //訪问根节点
                v.push_back(p->val);
                //根节点插入栈中,用来訪问右子树
                s.push(p);
                //遍历左子树
                p = p->left;
            }
            else{
                //左子树訪问完成。訪问右子树
                p = s.top();
                s.pop();
                p = p->right;
            }
        }
        return v;
    }
};

【代码三】

/*------------------------------------------------
*   日期：2015-03-25
*   作者：SJF0115
*   题目: 144.Binary Tree Preorder Traversal
*   来源：https://oj.leetcode.com/problems/binary-tree-preorder-traversal/
*   结果：AC
*   来源：LeetCode
*   博客：
------------------------------------------------------*/
#include <iostream>
#include <stack>
#include <vector>
using namespace std;

// 二叉树节点结构
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;
        if(root == nullptr){
            return result;
        }//if
        stack<TreeNode*> s;
        s.push(root);
        TreeNode *node;
        while(!s.empty()){
            node = s.top();
            s.pop();
            result.push_back(node->val);
            // 右子树
            if(node->right){
                s.push(node->right);
            }//if
            // 左子树
            if(node->left){
                s.push(node->left);
            }//if
        }//while
        return result;
    }
};
// 1.创建二叉树
void CreateTree(TreeNode* &root){
    int val;
    //按先序次序输入二叉树中结点的值,‘-1’表示空树
    cin>>val;
    // 空节点
    if(val == -1){
        root = nullptr;
        return;
    }//if
    root = new TreeNode(val);
    //构造左子树
    CreateTree(root->left);
    //构造右子树
    CreateTree(root->right);
}
int main() {
    freopen("C:\Users\Administrator\Desktop\c++.txt", "r", stdin);
    TreeNode* root = nullptr;
    vector<int> result;
    // 创建二叉树
    CreateTree(root);

    Solution solution;
    result = solution.preorderTraversal(root);
    for(int i = 0;i < result.size();++i){
        cout<<result[i]<<" ";
    }
    cout<<endl;
    return 0;
}