144. 二叉树的前序遍历

题目描述

解题思路

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res ;
        traverse(root, res);
        return res ;
    }

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

递归的实现就是：每一次递归调用都会把函数的局部变量、参数值和返回地址等压入调用栈中

递归的实质就是迭代

统一格式法

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res ;
        vector<TreeNode *> s ;
        if (root != NULL) s.push_back(root);
        while (!s.empty()) {
            TreeNode* last = s.back();
            s.pop_back();
            if (last == NULL) {
                TreeNode* last = s.back();
                s.pop_back();
                res.push_back(last->val);
            } else {
                s.push_back(last);
                s.push_back(NULL);
                if (last->right) s.push_back(last->right);
                if (last->left) s.push_back(last->left);
            }
        }
        return res ;
    }
};

学习感想

为什么当时没有用rust做呢

use std::rc::Rc;
use std::cell::RefCell;
impl Solution {
    pub fn preorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
        if let Some(r) = root {
            // use std::ops::DerefMut;
            // let mut ref_node: std::cell::RefMut<TreeNode> = r.borrow_mut();
            // let node: &mut TreeNode = &mut ref_node;
            let mut res: Vec<i32> = vec![r.borrow().val];
            res.append(&mut Self::preorder_traversal(r.borrow_mut().left.take()));
            res.append(&mut Self::preorder_traversal(r.borrow_mut().right.take()));
            res
        } else {
            vec![]
        }
    }
}