【leetcode刷题笔记】Binary Tree Preorder Traversal

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?

解题：应该是很简单的一道题，纠结了好久T_T

        基本思路很简单，用栈模拟就可以了。首先根节点压栈，每次弹出栈顶元素，并把它的值存入返回值向量中。如果它的右子树不为空，就把右子树根节点压栈，左子树不为空也把左子数根节点压栈，注意一定是右左这样的顺序。

        主要纠结在两个地方：

        1. 结构体指针的初始化：

struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));

　　注意要给指针分配空间，使用malloc需要包含头文件#include<cstdlib>

     2.每次压栈以后栈顶元素就变了，所以要先把栈顶元素pop到一个变量中，再继续后续操作。

代码如下：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 #include <iostream>
 #include <stack>
 #include <vector>
 #include <cstdlib>
 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) {
       stack<TreeNode*>s;
       vector<int>ans;
       if(root == NULL)
            return ans;
       s.push(root);
       while(!s.empty()){
            struct TreeNode* temp = s.top();
            s.pop();
            ans.push_back(temp->val);
            if(temp->right != NULL)
                s.push(temp->right);
            if(temp->left != NULL){
                s.push(temp->left);
            }
       }
        return ans;
    }
};
int main(){
    Solution so;
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode* root_l_c = (struct TreeNode*)malloc(sizeof(struct TreeNode));

    root->val = 2;
    root_l_c->val = 3;

    root_l_c->left = NULL;
    root_l_c->right = NULL;

    root->right = NULL;
    root->left = root_l_c;
    //cout << root->left->val<<endl;
    so.preorderTraversal(root);
}

 Java版本递归解法：

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> answer = new ArrayList<>();
        return preorderTraversalHelper(answer, root);
    }
    public List<Integer> preorderTraversalHelper(List<Integer> answer,TreeNode root) {
        if(root == null)
            return answer;
        answer.add(root.val);
        answer = preorderTraversalHelper(answer,root.left);
        answer = preorderTraversalHelper(answer,root.right);
        return answer;           
    }
}

一个小笔记是在eclipse里面使用List或者ArrayList的时候要加上

 import java.util.ArrayList;import java.util.List;