题解 | 二叉树的后序遍历 迭代

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        if (!root) return ans;

        stack<TreeNode*> stk;
        TreeNode *prev = nullptr;
        while (root || !stk.empty()) {
            while (root) {
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            if (!root->right || root->right == prev) {
                ans.emplace_back(root->val);
                prev = root;
                root = nullptr;
            } else {
                stk.push(root);
                root = root->right;
            }
        }
        return ans;
    }
};