【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;
    }
};