logseq_notes/pages/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_problems #difficulty_easy

  Additional tags:

  #leetcode #CS_list_need_practicing

  Revisions:

  N/A

Related topics:

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;
}
};