94. Binary Tree Inorder Traversal

https://www.cnblogs.com/grandyang/p/4297300.html

用堆来辅助，先存储所有的左节点，再根据左节点找右节点

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode*> container;
        while(root || !container.empty()){
            while(root){
                container.push(root);
                root = root->left;
            }
            root = container.top();
            container.pop();
            result.push_back(root->val);
            root = root->right;
        }
        return result;
    }
};