1 /***************************
2 https://leetcode.com/problems/binary-tree-preorder-traversal/
3 @date 2015.5.13
4 @description
5 用非递归方法对二叉树进行先序遍历
6 借助辅助栈
7 每次先访问根节点，把节点压入栈，再转向其左孩子，直至左子树的左孩子为空，依次将栈顶元素出栈，转向右孩子。
8
9
10 ****************************/
11
12 #include <iostream>
13 #include <stack>
14 #include <vector>
15
16 using namespace std;
17
18 struct TreeNode{
19 int val;
20 TreeNode *left, *right;
21 TreeNode(int x): val(x), left(NULL), right(NULL) {}
22 };
23
24 class Solution{
25 public:
26 vector<int> preorderTraversal(TreeNode* root) {
27 stack<TreeNode *> s;
28 vector<int> res;
29 if (!root)
30 return res;
31
32 while (root != NULL || !s.empty()){
33 while (root != NULL){
34 res.push_back(root->val);
35 s.push(root);
36 root = root->left;
37 }
38
39 if (!s.empty()){
40 root = s.top();
41 s.pop();
42 root = root->right;
43 }
44 }
45 return res;
46 }
47 };
48
49
50 // 第二种方法迭代先序遍历二叉树
51 vector<int> preorderTraversal(TreeNode *root){
52 vector<int> res;
53 if (!root)
54 return res;
55 stack<TreeNode *> s;
56 s.push(root);
57 while (!s.empty()){
58 TreeNode *temp = s.top();
59 s.pop();
60 res.push_back(temp->val);
61 if (temp->right) s.push(temp->right); // 先对右孩子入栈
62 if (temp->left) s.push(temp->left);
63 }
64 return res;
65 }
66
67 TreeNode *insert(TreeNode *root, int data){
68 TreeNode *ptr = root;
69 TreeNode *tempNode; // 存储的是插入节点的父节点
70 TreeNode *newNode = new TreeNode(data);
71
72 if (ptr == NULL)
73 return newNode;
74 else{
75 while (ptr != NULL){
76 tempNode = ptr;
77 if (ptr->val >= data){
78 ptr = ptr->left;
79 }else{
80 ptr = ptr->right;
81 }
82 }
83 if (tempNode->val >= data){
84 tempNode->left = newNode;
85 }else{
86 tempNode->right = newNode;
87 }
88 }
89 return root;
90 }
91
92 // 递归先序遍历二叉树
93 void travPre(TreeNode *root){
94 if (!root) return;
95 cout << root->val << " ";
96 travPre(root->left);
97 travPre(root->right);
98 }
99
100
101 int main(){
102 TreeNode *root = NULL;
103 int temp = 0;
104 cin >> temp;
105 while (temp != 0){ // 以0结尾（输入0终止）
106 root = insert(root, temp);
107 cin >> temp;
108 } // 创建一棵二叉树
109
110 // 递归先序遍历
111 // travPre(root);
112 Solution a;
113 vector<int> res = preorderTraversal(root);
114 for (vector<int>::iterator it = res.begin(); it != res.end(); ++it)
115 cout << *it << " ";
116
117 }
