二叉树的前序遍历

/**

 * Definition for a binary tree node.

 * 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> vec;

        preOrder(root, vec);

        return vec;

    }

    //递归写法

    /*

    void preOrder(TreeNode *root, vector<int> &path)

    {

        if (root)

        {

            path.push_back(root->val);

            preOrder(root->left, path);

            preOrder(root->right, path);

        }

    }

    */

    //非递归写法

    void preOrder(TreeNode *root, vector<int> &path)

    {

        stack<TreeNode *> TreeNodeStack;

         

        while (root != NULL || !TreeNodeStack.empty())

        {

            while (root != NULL)

            {

                path.push_back(root->val);

                TreeNodeStack.push(root);

                root = root->left;

            }

            if (!TreeNodeStack.empty())

            {

                root = TreeNodeStack.top();

                TreeNodeStack.pop();

                root = root->right;

            }

        }

    }

};