94. Binary Tree Inorder Traversal (C++)

94 Binary Tree Inorder Traversal

Description

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

Follow up: Recursive solution is trivial, could you do it iteratively?

---

My Solution

• recursive

  /**
   * Definition for a binary tree node.
   * struct TreeNode {
   *     int val;
   *     TreeNode *left;
   *     TreeNode *right;
   *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
   * };
   */
  
  class Solution{
    public:
        vector inorderTraversal(TreeNode* root){
        vector ans;
        inorder(root, ans);
        return ans;
      }
    
        void inorder(TreeNode* root, vector &ans){
        if (root){
          inorder(root->left, ans);
          ans.push_back(root->val);
          inorder(root->right, ans);
        }
      }
  }
  
  
  

• iterative

  class Solution {
  public:
    vector inorderTraversal(TreeNode* root) {
        vector ans;
        inorder(root, ans);
        return ans;
    }
    void inorder(TreeNode* root, vector &ans){
        if (!root){
            return;
        }
        stack s;
        while(!s.empty() || root){
            if (root){
                s.push(root);
                root = root->left;
            }else{
                root = s.top();
                ans.push_back(root->val);
                s.pop();
                root = root->right;
            }
        }
    }