Leetcode

2022-05-11

1. 二叉树的前序遍历

 * 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) {
        std::vector<int> res;
        
        // traversal(root, res);
        not_traversal(root, res);

        return res;
    }

    void traversal(TreeNode* node, std::vector<int>& res) {
        if (!node) {
            return;
        }
        res.push_back(node->val);
        traversal(node->left, res);
        traversal(node->right, res);
    }

    void not_traversal(TreeNode* node, std::vector<int>& res) {
        std::stack<TreeNode*> s;

        s.push(node);
        while(!s.empty()) {
            TreeNode* cur = s.top();
            s.pop();
            if (!cur) {
                continue;
            }
            res.push_back(cur->val);
            if (cur->right != nullptr) {
                s.push(cur->right);
            }

            if (cur->left != nullptr) {
                s.push(cur->left);
            }
        }
    }
};

2. 二叉树的中序遍历