前中后遍历 递归版
1 /* Recursive solution */
2 class Solution {
3 public:
4 vector<int> preorderTraversal(TreeNode *root) {
5
6 vector<int> result;
7 preorderTraversal(root, result);
8
9 return result;
10 }
11
12 void preorderTraversal(TreeNode *root, vector<int>& result)
13 {
14 if(root == NULL) return;
15 result.push_back(root->val);
16
17 preorderTraversal(root->left, result);
18 preorderTraversal(root->right, result);
19 }
20
21 };
1 /* Recursive solution */
2 class Solution {
3 public:
4 vector<int> inorderTraversal(TreeNode *root) {
5
6 vector<int> result;
7 inorderTraversal(root, result);
8
9 return result;
10 }
11
12 void inorderTraversal(TreeNode *root, vector<int>& result)
13 {
14 if(root == NULL) return;
15
16 inorderTraversal(root->left, result);
17 result.push_back(root->val);
18 inorderTraversal(root->right, result);
19 }
20
21 };
1 /* Recursive solution */
2 class Solution {
3 public:
4 vector<int> postorderTraversal(TreeNode *root) {
5
6 vector<int> result;
7 postorderTraversal(root, result);
8
9 return result;
10 }
11
12 void postorderTraversal(TreeNode *root, vector<int>& result)
13 {
14 if(root == NULL) return;
15
16 postorderTraversal(root->left, result);
17 result.push_back(root->val);
18 postorderTraversal(root->right, result);
19 }
20
21 };
下面是迭代版本
1 preorder： 节点入栈一次， 入栈之前访问。
2 inorder：节点入栈一次，出栈之后访问。
3 postorder：节点入栈2次，第二次出战后访问。
1 class Solution {
2 public:
3 vector<int> preorderTraversal(TreeNode *root) {
4
5 vector<int> result;
6 stack<TreeNode*> stack;
7
8 TreeNode *p = root;
9
10 while( NULL != p || !stack.empty())
11 {
12 while(NULL != p)
13 {
14 result.push_back(p->val);
15
16 stack.push(p);
17 p = p->left;
18 }
19
20 if(!stack.empty())
21 {
22 p= stack.top();
23 stack.pop();
24
25 p=p->right;
26 }
27 }
28
29 return result;
30 }
31
32 };
1 class Solution {
2 public:
3 vector<int> inorderTraversal(TreeNode *root) {
4
5 vector<int> result;
6 stack<TreeNode*> stack;
7
8 TreeNode *p = root;
9
10 while( NULL != p || !stack.empty())
11 {
12 while(NULL != p)
13 {
14 stack.push(p);
15 p = p->left;
16 }
17
18 if(!stack.empty())
19 {
20 p= stack.top();
21 stack.pop();
22
23 result.push_back(p->val);
24
25 p=p->right;
26 }
27 }
28
29 return result;
30 }
31
32 };
后续， 用bStack标记是否是第一次访问，如果是第一次访问，再次入栈，否则直接访问，记得将P = NULL；
1 class Solution {
2 public:
3 vector<int> postorderTraversal(TreeNode *root) {
4
5 vector<int> result;
6 stack<TreeNode*> stack;
7 std::stack<bool> bStack;
8
9 TreeNode *p = root;
10 bool isFirst;
11
12 while( NULL != p || !stack.empty())
13 {
14 while(NULL != p)
15 {
16 stack.push(p);
17 bStack.push(false);
18 p = p->left;
19 }
20
21 if(!stack.empty())
22 {
23 p= stack.top();
24 stack.pop();
25
26 isFirst = bStack.top();
27 bStack.pop();
28
29 if(isFirst == 0)
30 {
31 stack.push(p);
32 bStack.push(true);
33 p=p->right;
34 }
35 else
36 {
37 result.push_back(p->val);
38 p = NULL;
39 }
40
41 }
42 }
43
44 return result;
45 }
46
47 };
[LeetCode] Binary Tree Preorder/Inorder/Postorder
Traversal
时间： 2024-10-11 06:09:18