leetcode 144.二叉树的前序遍历

//解法一：非递归版本
Time:O(n),Space:O(n)
/**
 * 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) 
    {
        vector<int> v;
        stack<TreeNode*> s;

        if(root!=NULL)
        {
            s.push(root);

            while(!s.empty())
            {
                TreeNode* node=s.top();
                s.pop();
                v.push_back(node->val);

                if(node->right!=NULL) s.push(node->right);
                if(node->left!=NULL) s.push(node->left);
            }
        }

        return v;
    }
};

//解法二：递归版本
//Time:O(n) Space:O(n)
/**
 * 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) 
    {
        vector<int> v;
        
        preorder(root,v);

        return v;
    }

    void preorder(TreeNode* root,vector<int>& v)
    {
         if(root!=NULL)
         {
             v.push_back(root->val);
             preorder(root->left,v);
             preorder(root->right,v);
         }
    }
};

//解法三：非递归版 推荐写法
//Time:O(n),Space:O(n)
/**
 * 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) 
    {
        vector<int> ret;
        stack<TreeNode*> s;

        while(root!=NULL||!s.empty())
        {
            while(root!=NULL)
            {
                ret.push_back(root->val);
                s.push(root);
                root=root->left;
            }

            root=s.top();
            s.pop();
            root=root->right;
        }

        return ret;
    }
};

来源：CSDN

作者：weixin_42136255

链接：https://blog.csdn.net/weixin_42136255/article/details/103657111

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前序遍历