BinaryTreeInorderTraversal.cpp

// // 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) {}
// };

// https://shubo.io/iterative-binary-tree-traversal/

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if (!root) return res;

        stack<TreeNode*> s;
        TreeNode* cur = root;
        while(cur || !s.empty()) {
            if (cur) {
                s.push(cur);
                cur = cur -> left;
            }
            else {
                TreeNode* node = s.top();
                s.pop();
                res.push_back(node -> val);
                cur = node -> right;
            }
        }
        return res;
    }
};