算法学习之运用栈模拟递归-二叉树的后序遍历

题目要求：

给定一个二叉树，返回它的 后序 遍历。

示例:

输入: [1,null,2,3]
1
\
2
/
3

输出: [3,2,1]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/binary-tree-postorder-traversal

解题代码：

//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) {}
};

struct Command {
    string s; // go, print
    TreeNode* node;
    Command(string s, TreeNode* node): s(s), node(node) {}
};

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) { // 后序遍历
        vector<int> res;
        if (root == NULL)
            return res;

        stack<Command> stack;
        stack.push(Command("go", root));
        while (!stack.empty()) {
            Command command = stack.top();
            stack.pop();

            if (command.s == "print")
                res.push_back(command.node->val);
            else {
                assert(command.s == "go");
                // 实际执行，先推入左孩子，再推入右孩子，最后打印
                stack.push(Command("print", command.node));
                if (command.node -> right)
                    stack.push(Command("go", command.node->right));
                if (command.node -> left)
                    stack.push(Command("go", command.node->left));

            }
        }
        return res;
    }
};