[leetcode] Binary Tree Preorder Traversal_prepreordertraversal' was not declared in this sco-优快云博客

本文链接：https://blog.youkuaiyun.com/lydyangliu/article/details/37959829

本文介绍二叉树的前序遍历算法，包括递归和非递归两种实现方式，并提供详细的代码示例。

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使用递归

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> res;
public:
    vector<int> preorderTraversal(TreeNode *root) {
        if (root!=NULL) {
            res.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return res;
    }
};

非递归

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {

private:
    vector<int> res;
    stack<TreeNode*> stk;
public:
    vector<int> preorderTraversal(TreeNode *root) {
        if (root!=NULL) {
            stk.push(root);
            while (!stk.empty()) {
                TreeNode* node=stk.top();
                stk.pop();
                res.push_back(node->val);
                if (node->right) {//先把右孩子放入栈中
                    stk.push(node->right);
                }
                if (node->left) {
                    stk.push(node->left);
                }
             
            }
        }
        
        return res;
    }

};

通用模板：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> res;
    stack<TreeNode*> stk;
public:
    vector<int> preorderTraversal(TreeNode *root) {
        if (root==NULL) {
            return res;
        }
        stk.push(root);
        while (!stk.empty()) {
            TreeNode *tmp=stk.top();
            if (tmp->left==NULL&&tmp->right==NULL) {
                stk.pop();
                res.push_back(tmp->val);
            }else{
                stk.pop();
                
                if (tmp->right!=NULL) {
                    stk.push(tmp->right);
                }
                
                if (tmp->left!=NULL) {
                    stk.push(tmp->left);
                }
                stk.push(tmp);
                tmp->left=tmp->right=NULL;
            }
            
        }
        return res;
    }
};