leetcode Binary Tree Preorder Traversal

实现前序遍历。可参见中序遍历Binary Tree Inorder Traversal

递归：

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    void helper144(TreeNode *root, vector<int> &perm)

    {

        if (!root) return;

        perm.push_back(root -> val);

        helper144(root -> left, perm);

        helper144(root -> right, perm);

    }

    vector<int> preorderTraversal(TreeNode *root) 

    {

        vector<int> perm;

        helper144(root, perm);

        return perm;

    }

};

 

非递归：

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    vector<int> preorderTraversal(TreeNode *root) 

    {

        vector<int> perm;

        stack<TreeNode *> sta;

        TreeNode *p = root;

        

        while(!sta.empty() || p)

        {

            while(p)

            {

                perm.push_back(p->val);

                if (p -> right)

                    sta.push(p -> right);

                p = p -> left;

            }

            if (!sta.empty())

            {

                p = sta.top();

                sta.pop();

            }

        }

        return perm;

    }

};