145. Binary Tree Postorder Traversal.cpp

// Iterative
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int>res;
        stack<TreeNode*>s;
        while(!s.empty() || root){
            if(root){
                s.push(root->left);
                res.push_back(root->val);
                root = root->right;
            }
            else{
                root = s.top();
                s.pop();
            }
        }
        reverse(res.begin(), res.end());
        return res;
    }
};

// Recursive
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int>res;
        DFS(root, res);
        return res;
    }
    
    void DFS(TreeNode* root, vector<int>& res){
        if(!root) return;
        DFS(root->left, res);
        DFS(root->right, res);
        res.push_back(root->val);
    }
};