二叉树基础算法--遍历

• 二叉树的前序遍历(144. Binary Tree Preorder Traversal)

递归

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void perorderTraversalHelper(vector<int> &vec, TreeNode *root)
13     {
14         if (root == NULL)
15         {
16             return;
17         }
18
19         vec.push_back(root->val);
20         perorderTraversalHelper(vec, root->left);
21         perorderTraversalHelper(vec, root->right);
22     }
23
24     vector<int> preorderTraversal(TreeNode* root) {
25         vector<int> vec;
26         perorderTraversalHelper(vec, root);
27         return vec;
28     }
29 };

非递归

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> preorderTraversal(TreeNode* root) {
13         vector<int> vec;
14
15         TreeNode *p = root;
16         stack<TreeNode *> nodeStack;
17
18         while (p != NULL || !nodeStack.empty())
19         {
20             while (p != NULL)
21             {
22                 vec.push_back(p->val);
23                 nodeStack.push(p);
24                 p = p->left;
25             }
26
27             if (!nodeStack.empty())
28             {
29                 TreeNode *node = nodeStack.top();
30                 nodeStack.pop();
31                 p = node->right;
32             }
33         }
34
35         return vec;
36     }
37 };

• 二叉树的中序遍历(94. Binary Tree Inorder Traversal)

递归

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void inorderTraversalHelper(vector<int> &vec, TreeNode *root)
13     {
14         if (root == NULL)
15         {
16             return;
17         }
18
19         inorderTraversalHelper(vec, root->left);
20         vec.push_back(root->val);
21         inorderTraversalHelper(vec, root->right);
22     }
23
24     vector<int> inorderTraversal(TreeNode* root) {
25         vector<int> vec;
26         inorderTraversalHelper(vec, root);
27         return vec;
28     }
29 };

非递归

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> inorderTraversal(TreeNode* root) {
13
14         vector<int> vec;
15
16         TreeNode *p = root;
17         stack<TreeNode *> nodeStack;
18
19         while (p != NULL || !nodeStack.empty())
20         {
21             while (p != NULL)
22             {
23                 nodeStack.push(p);
24                 p = p->left;
25             }
26
27             if (!nodeStack.empty())
28             {
29                 TreeNode *node = nodeStack.top();
30                 nodeStack.pop();
31                 vec.push_back(node->val);
32
33                 p = node->right;
34             }
35         }
36
37         return vec;
38     }
39 };

• 二叉树的后序遍历

递归

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void postorderTraversalHelper(vector<int> &vec, TreeNode *root)
13     {
14         if (root == NULL)
15         {
16             return;
17         }
18
19         postorderTraversalHelper(vec, root->left);
20         postorderTraversalHelper(vec, root->right);
21         vec.push_back(root->val);
22     }
23
24     vector<int> postorderTraversal(TreeNode* root) {
25         vector<int> vec;
26         postorderTraversalHelper(vec, root);
27         return vec;
28     }
29 };