树的各种遍历实现

树的遍历有前序，后序，广度，以及中序遍历。下边将利用递归以及非递归的方式实现这几种遍历方式。

1.前序遍历

递归写法：

 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> res;
14         Traversal(root, res);
15         return res;
16     }
17     
18     void Traversal(TreeNode *root, vector<int> &res)
19     {
20         if(!root) return ;
21         res.push_back(root->val);
22         Traversal(root->left, res);
23         Traversal(root->right, res);
24     }
25 };

非递归写法： 

 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> res;
14         if(!root) return res;
15         stack<TreeNode *>stk;
16         stk.push(root);
17         
18         while(!stk.empty()){
19             TreeNode* s = stk.top();
20             stk.pop();
21             res.push_back(s->val);
22             if(s->right) stk.push(s->right);
23             if(s->left) stk.push(s->left);
24         }
25         
26         return res;
27     }
28 };

非递归写法主要要弄清楚树遍历的顺序，这样就很容易写出来相应的stack的压出栈顺序。

2.中序遍历

递归写法

 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         vector<int> res;
14         Traversal(root, res);
15         return res;
16     }
17     
18     void Traversal(TreeNode *root, vector<int> &res)
19     {
20         if(!root) return ;
21         Traversal(root->left, res);
22         res.push_back(root->val);
23         Traversal(root->right, res);
24     }
25 };

 非递归写法

 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         vector<int> res;
14         if(!root) return res;
15         TreeNode *cur = root;
16         stack<TreeNode *>stk;
17         
18         while(cur || !stk.empty()){
19             if(cur){
20                 stk.push(cur);
21                 cur = cur->left;
22             }else{
23                 cur = stk.top();
24                 stk.pop();
25                 res.push_back(cur->val);
26                 cur = cur->right;
27             }
28         }
29         
30         return res;
31     }
32 };

非递归写法要注意stack弹出栈的操作，这里的思路是这样的：当左边没有元素时弹出，但是弹出后如果右边不是空则压入右边，如果右边为空则继续弹出，也就成了以上所写的代码了。这里无法使用前序遍历那种仅仅根据压栈出栈的顺序写stack的原因在于前序本质还是从上至下的，不会产生重复遍历的过程。而中序从下至上，如果仅仅push(left), push(mid),push(right)这种方法会导致重复访问节点的问题。

3.后序遍历

递归写法

 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> postorderTraversal(TreeNode* root) {
13         vector<int> res;
14         Traversal(root, res);
15         return res;
16     }
17     
18     void Traversal(TreeNode *root, vector<int> &res)
19     {
20         if(!root) return ;
21         Traversal(root->left, res);
22         Traversal(root->right, res);
23         res.push_back(root->val);
24     }
25 };

非递归写法： 

 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> postorderTraversal(TreeNode* root) {
13         TreeNode *cur = root;
14         vector<int> res;
15         if(!root) return res;
16         stack<TreeNode *>stk;
17         
18         stk.push(root);
19         while(!stk.empty()){
20             TreeNode *t = stk.top();
21             stk.pop();
22             res.push_back(t->val);
23             if(t->left)
24                 stk.push(t->left);
25             if(t->right)
26                 stk.push(t->right);
27         }
28         reverse(res.begin(), res.end());
29         return res;
30     }
31 };

非递归写法利用前序遍历的思想进行编写，然后进行reverse。因为前序遍历中弹出的条件更容易判断，用完就可以弹出。所以利用前序遍历更容易

4.广度搜索

 

 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<vector<int>> levelOrder(TreeNode* root) {
13         vector<vector<int> >res;
14     
15         queue<TreeNode *> q;
16         if(!root) return res;
17         q.push(root);
18         
19         while(!q.empty()){
20             int count = q.size();
21             vector<int> r;
22             while(count){
23                 TreeNode *t = q.front();
24                 r.push_back(t->val);
25                 q.pop();
26                 if(t->left) q.push(t->left);
27                 if(t->right) q.push(t->right);
28                 count--;
29             }
30             res.push_back(r);
31         }
32         
33         return res;
34     }
35 };

利用队列实现。

 

 

posted @ 2015-09-02 17:50  qtalker  阅读(244)  评论(0)  编辑  收藏  举报