二叉树的前序、中序、后序遍历（迭代）

二叉树的通用中序遍历方法（迭代）：
思路来源：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/solution/yan-se-biao-ji-fa-yi-chong-tong-yong-qie-jian-ming/

/**
 * 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) {
        stack<pair<TreeNode*, int>> st;
        vector<int> ans;
        st.push({root,0});
        while(!st.empty()){
            auto node = st.top().first;
            auto color = st.top().second;
            st.pop();
            if(node == nullptr){
                continue;
            }
            if(color == 0){
                st.push(make_pair(node->right,0));
                st.push(make_pair(node,1));
                st.push(make_pair(node->left,0));
            }else{
                ans.push_back(node->val);
            }
        }
        return ans;
    }
};