LeetCode 145 二叉树的后续遍历 <双百>

Github-145-源码 给定一个二叉树，返回它的 后序 遍历。

迭代版本,使用栈保存先前节点,最后逆序输出

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<TreeNode*> stack ;
        vector<int> output;
        if (root == nullptr) {
            return output;
        }

        stack.push(root);
        while (!stack.empty()) {
            TreeNode *node = stack.top();
            stack.pop();
            output.push_back(node->val);
            if (node->left != nullptr) {
                stack.push(node->left);
            }
            if (node->right != nullptr) {
                stack.push(node->right);
            }
        }
        reverse(output.begin(),output.end());
        return output;
    }
};