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Binary Tree Inorder/Preorder Traversal 返回中序和前序/遍历二叉树的元素集合

给定一个二叉树,以集合方式返回其中序/先序方式遍历的所有元素。

有两种方法,一种是经典的中序/先序方式的经典递归方式,另一种可以结合栈来实现非递归

 

Given a binary tree, return the inorder traversal of its nodes‘ values.

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

   1         2    /   3

 

return [1,3,2].


OJ‘s Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where ‘#‘ signifies a path terminator where no node exists below.

Here‘s an example:

   1  /  2   3    /   4         5
The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".
 
 1 /** 2  * Definition for binary tree 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         vector<int> ret;14         if(root == NULL)15             return ret;16             17         stack<TreeNode *> stack;18         stack.push(root);19         20         while(!stack.empty()){21             TreeNode *node = stack.top();22             stack.pop();23             if(node->left == NULL && node->right == NULL){24                 ret.push_back(node->val);25             }26             else{27                 if(node->right != NULL)28                     stack.push(node->right);29                 stack.push(node);30                 if(node->left != NULL)31                     stack.push(node->left);32                     33                 node->left = node->right = NULL;34             }35             36         }37         38         return ret;39             40     }41 };

 

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

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

   1         2    /   3

 

return [1,2,3].

和上面要求一样,只是要返回以中序方式序列的元素,这次用递归实现:

 1 /** 2  * Definition for binary tree 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> ret;14         if(root == NULL)15             return ret;16         PreorderTraversal(root,ret);17         return ret;18     }19     20     void PreorderTraversal(TreeNode *root,vector<int> &ret){21         if(root != NULL){22             ret.push_back(root->val);23             PreorderTraversal(root->left,ret);24             PreorderTraversal(root->right,ret);25         }26     }27 };

 

Binary Tree Inorder/Preorder Traversal 返回中序和前序/遍历二叉树的元素集合