二叉树的中序遍历非递归

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    vector<int> inorderTraversal(TreeNode* root) {
        // write code here
        // 直接奥义非递归
        vector<int> res;
        if(root == nullptr) return res;
        stack<TreeNode*> s;
        while(root || s.size()) {
            while(root) {
                s.push(root);
                root = root->left;
            }
            root = s.top();
            s.pop();
            res.push_back(root->val);
            root = root->right;
        }
        return res;
    }
};