leetcode/DS_Tree/94+inorderTraversal_2022.cpp at master · DuanYaQi/leetcode · GitHub

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/**
 * 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) {
        vector<int> res;
        stack<TreeNode*> st;
        if (root != nullptr) st.push(root);

        while(!st.empty()) {
            TreeNode* node = st.top();
            if (node != nullptr) {
                st.pop();
                if (node->right != nullptr) st.push(node->right);
                st.push(node);
                st.push(nullptr);
                if (node->left != nullptr) st.push(node->left);
            } else {
                st.pop();
                node = st.top();
                res.push_back(node->val);
                st.pop();
            }
        }
        return res;
    }
};