94. Binary Tree Inorder Traversal

• 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 {
private:
    vector<int> res;
public:
    vector<int> inorderTraversal(TreeNode* root)
    {
        dfs(root);
        return res;
    }

    void dfs(TreeNode* root)
    {
        if(!root) return;

        dfs(root->left);
        res.push_back(root->val);
        dfs(root->right);
    }
};

• T: $O(N)$
• S: $O(N)$

Hint - Iteration

/**
 * 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> inorderTraversal(TreeNode* root)
    {
        // Stack to help with traversal
        // Vector to store the result of the inorder traversal

        // Start with the root node

        // Traverse the tree
        // Continue until all nodes are processed
        while ()
        {
            // Reach the leftmost node of the current node
            while ()
            {
                // Push the current node onto the stack
                // Move to the left child
            }
            // Process the node
            // Get the node from the top of the stack
            // Remove the node from the stack
            // Add the node's value to the result

            // Move to the right child of the node
        }
        // Return the vector containing the inorder traversal
    }
};

• Iteration

/**
 * 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> inorderTraversal(TreeNode* root)
    {
        stack<TreeNode*> stk;
        vector<int> res;

        TreeNode* node = root;

        while (node || !stk.empty())
        {
            while (node)
            {
                stk.push(node);
                node = node->left;
            }
            node = stk.top(); stk.pop();
            res.push_back(node->val);
            node = node->right;
        }
        return res;
    }
};

• T: $O(N)$
• S: $O(N)$