Binary Tree Preorder Traversal

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

Example:

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

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

typedef char ElemType;

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

Binary Tree Preorder Traversal

void TraverseBiTree(BiTree T){
    if(T == NULL)
        return;
    printf("%c",T->data);
    TraverseBiTree(T->left);
    TraverseBiTree(T->right);
}

//非递归方法待完善,以下方法便于理解，如果还是不太理解，不妨画图来看一下，见得多了，抽象能力自然就有了。enjoy!

Binary Tree Preorder Traversal
这个方法便于记忆。
class Solution{
    public:
        vector<int> preorderTraversal(TreeNode* root){
            if(!root) return {};
            vector<int> res;
            stack<TreeNode*> s{{root}};
            while(!s.empty()){
                TreeNode* t = s.top();s.pop();
                res.push_back(t->val);
                if(t->right) s.push(t->right);
                if(t->left) s.push(t->left);
            }
            return res;
        }
};

//这个方法可以拓展思路
c++ solution2:
class Solution{
    public:
        vector<int> preorderTraversal(TreeNode* root){
            vector<int> res;
            stack<TreeNode*> s;
            TreeNode *p = root;
            while(!s.empty() || p){
                if(p){
                    s.push(p);
                    res.push_back(p->val);
                    p = p->left;
                }
                else{
                    TreeNode *t = s.top();s.pop();
                    p = t->right;
                }
            }
            return res;
        }
};