【Leetcode】【Medium】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].

解题思路：

二叉树非递归先序遍历，使用栈来保存被遍历到的，但是还没遍历其右子树的结点。

代码：

 /**
  * 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> vals;
         stack<TreeNode*> nodes;
         TreeNode* node = root;

         while (node != NULL || !nodes.empty()) {
             while (node) {
                 nodes.push(node);
                 vals.push_back(node->val);
                 node = node->left;
             }

             node = nodes.top();
             nodes.pop();
             node = node->right;
         }

         return vals;
     }
 };