[LeetCode]Binary Tree Preorder Traversal

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

For example: Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

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

思考：不可以递归，那么用栈存储结点。

先用递归法做，算是加深记忆。需要注意ret需要清空，不然会记录上一次测试用例，所以将先序遍历单独写出来。

class Solution {
private:
	vector<int> ret;
public:
	void preorder(TreeNode *root)
	{
		if(root) 
		{
			ret.push_back(root->val);
			preorder(root->left);
			preorder(root->right);
		}
	}
    vector<int> preorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
		ret.clear();
		preorder(root);
		return ret;
    }
};

非递归法：用栈存储遍历过的结点，先右子树后子树。

class Solution {
private:
	vector<int> ret;
public:
    vector<int> preorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
		ret.clear();
		if(!root) return ret;
		stack<TreeNode *> s;
		s.push(root);		
		while(!s.empty())
		{
			TreeNode *cur=s.top();
			s.pop();
			ret.push_back(cur->val);
			if(cur->right)
				s.push(cur->right);
			if(cur->left) 
				s.push(cur->left);					
		}
		return ret;
    }
};