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* node) {
        if (!node) return;
        dfs(node->left);
        res.emplace_back(node->val);
        dfs(node->right);
    }
};

• T: $O(N)$
• S: $O(N)$

Iteration

class Solution {
private:
    vector<int> res;
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.emplace_back(node->val);
            node = node->right;
        }
        return res;
    }
};

• T: $O(N)$
• S: $O(N)$