binary-tree-preorder-traversal 迭代法求解二叉树前序遍历

 1 /**
 2  * struct TreeNode {
 3  *    int val;
 4  *    struct TreeNode *left;
 5  *    struct TreeNode *right;
 6  * };
 7  */
 8 
 9 class Solution {
10 public:
11     /**
12      * 
13      * @param root TreeNode类 
14      * @return int整型vector
15      */
16     vector<int> preorderTraversal(TreeNode* root) {
17         vector<int> list;
18         stack<TreeNode*> st;
19         if(root == NULL)
20             return list;
21         st.push(root);
22         while( !st.empty()) {
23             TreeNode* temp = st.top();
24             st.pop();
25             list.push_back(temp->val);
26             if(temp->right != NULL)
27                 st.push(temp->right);
28             if(temp->left != NULL)
29                 st.push(temp->left);
30         }
31         return list;
32     }
33 };