Binary Tree postorder Traversal

Given a binary tree, return the postordertraversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    
     2
    /
   3

Output: [3,2,1]

typedef char ElemType;

typedef struct BiTreeNode
{
    ElemType data;
    struct BiTreeNode *left;
    struct BiTreeNode *right;
}BiTreeNode,*BiTree;

Binary Tree PostOrder Traversal
void postOrderBiTree(BiTree T){
    if(T == NULL)
        return;
    
    postOrderBiTree(T->left);
    postOrderBiTree(T->right);
    printf("%c",T->data);
}

 非递归的方法，以下第一方法便于理解与记忆。第二个方法不便于记忆；

优先记忆此方法：


C++ Solution1:
class Solution{
    public:
        vector<int> postorderTraversal(TreeNode* root){
            if(!root) return{};
            vector<int> res;
            stack<TreeNode*> s{{root}};
            while(!s.empty()){
                TreeNode* t = s.top();s.pop();
                res.insert(res.begin(),t->val);
                if(t->left) s.push(t->left);
                if(t->right) s.push(t->right);
            }
            
            return res;
        }
};


c++ solution2:
class Solution{
    public:
        vector<int> postorderTraversal(TreeNode* root){
            vector<int> res;
            stack<TreeNode*> s;
            TreeNode *p = root;
            
            while(!s.empty() || p){
                if(p){
                    s.push(p);
                    res.insert(res.begin(),p->val);
                    p = p->right;
                }
                else{
                    TreeNode *t = s.top();s.pop();
                    p = t->left;
                }
            }
            return res;
        }
};