LeetCode144:Binary Tree Preorder Traversal

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

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

return [1,2,3].

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

先序遍历应该是三种遍历方式中最好使用非递归来实现的了。
可以每次访问一个节点后再将其右子节点入栈，然后左子节点入栈。然后再从栈中取元素，可以发现就是先序遍历的结果。需要注意的是如果子节点不存在就不需要进行入栈操作。
runtime:4ms

/** * 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> res;        if(root==NULL)            return res;        stack<TreeNode *> s;        TreeNode * node=root;        s.push(node);        while(!s.empty())        {            node=s.top();            s.pop();            res.push_back(node->val);            if(node->right)  s.push(node->right);            if(node->left)  s.push(node->left);        }        return res;    }};