leetcode

144. 二叉樹的前序遍歷

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9
 
  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     vector<int> preorderTraversal(TreeNode* root) {
15         vector<int>ans;
16         stack<TreeNode*>st;
17         if(!root)return ans;
 
18         st.push(root);
19         while(st.size())
20         {
21             auto p=st.top();
22             ans.push_back(p->val);
23             st.pop();
24             if(p->right)st.push(p->right);
25             if(p->left)st.push(p->left);
26         }
27         return ans;
 
28     }
29 };

94. 二叉樹的中序遍歷

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
         vector<int>ans;
         if(!root)return ans;
         stack<TreeNode*>st;
         auto cur=root;
         while(st.size()||cur)
         {
             while(cur)
             {
                 st.push(cur);
                 cur=cur->left;
             }
             auto p=st.top();
             st.pop();
             ans.push_back(p->val);
             cur=p->right;
         }

         return ans;
    }
};

145. 二叉樹的後序遍歷

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     vector<int> postorderTraversal(TreeNode* root) {
15         vector<int>ans;
16         if(!root)return ans;
17         stack<TreeNode*>st;
18         TreeNode* pre=nullptr,*cur=root;
19         
20         while(st.size()||cur)
21         {
22             while(cur)
23             {
24                 st.push(cur);
25                 cur=cur->left;
26             }
27             auto p=st.top();
28             if(!p->right||p->right==pre)
29             {
30                 ans.push_back(p->val);
31                 st.pop();
32                 pre=p;
33             }
34             else cur=p->right;
35         }
36         return ans;
37     }
38 };