notes/OJ notes/pages/Leetcode Binary-Tree-Postorder-Traversal.md

Leetcode Binary-Tree-Postorder-Traversal

2022-07-04 21:31

Algorithms:

#algorithm #DFS #DFS_postorder

Data structures:

#DS #binary_tree

Difficulty:

#coding_problem #difficulty-

Additional tags:

#leetcode #CS_list_need_practicing

Revisions:

N/A

Related topics:

tag:#DFS

• Leetcode Binary-Tree-Inorder-Traversal
• Leetcode Binary-Tree-Preorder-Traversal
• Leetcode Insert-Into-a-Binary-Search-Tree
• Leetcode Invert-Binary-Tree
• Leetcode Maximum-Depth-Of-Binary-Tree
• Leetcode Path-Sum
• Leetcode Search-In-a-Binary-Tree
• Leetcode Symmetric-Tree
• Leetcode Validate-Binary-Search-Tree

Links:

• Link to problem

---

Problem

https://leetcode.com/problems/binary-tree-postorder-traversal/

Examples

Example 1:

Input: root = [1,null,2,3] Output: [3,2,1]

Example 2:

Input: root = [] Output: []

Example 3:

Input: root = [1] Output: [1]

Constraints

• The number of the nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Thoughts

Same as Leetcode Binary-Tree-Inorder-Traversal and Leetcode Binary-Tree-Preorder-Traversal

== TODO: write iteration and another algo #TODO ==

Solution

Recursion

/**
 * 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 {
  void postorder(TreeNode *root, vector<int> &answer) {
    if (root == nullptr) {
      return;
    }

    postorder(root->left, answer);
    postorder(root->right, answer);
    answer.push_back(root->val);
  }

public:
  vector<int> postorderTraversal(TreeNode *root) {
    vector<int> answer;
    postorder(root, answer);
    return answer;
  }
};