Leetcode–refresh–binary Tree Pre Order traversal

Same with Inorder.

1. Recursive:

1 /**2 * Definition for binary tree3 * struct TreeNode {4 * int val;5 * TreeNode *left;6 * TreeNode *right;7 * TreeNode (int x): Val (x), left (null), right (null) {}8  * };9  */Ten classSolution { One  Public: A     voidGettree (TreeNode *root, vector<int> &result) { -         if(!root)return; -Result.push_back (root->val); theGettree (root->Left , result); -Gettree (root->Right , result); -     } -vector<int> Preordertraversal (TreeNode *root) { +vector<int>result; - Gettree (root, result); +         returnresult; A     } at};

2. Directly use stack

1 /**2 * Definition for binary tree3 * struct TreeNode {4 * int val;5 * TreeNode *left;6 * TreeNode *right;7 * TreeNode (int x): Val (x), left (null), right (null) {}8  * };9  */Ten classSolution { One  Public: Avector<int> Preordertraversal (TreeNode *root) { -vector<int>result; -         if(!root)returnresult; theStack<treenode *>s; -          while(Root | |!)S.empty ()) { -             if(root) { -S.push (root->Right ); +Result.push_back (root->val); -root = root->Left ; +}Else { ARoot =s.top (); at S.pop (); -             } -         } -         returnresult; -     } -};

3. Pointers:

1 /**2 * Definition for binary tree3 * struct TreeNode {4 * int val;5 * TreeNode *left;6 * TreeNode *right;7 * TreeNode (int x): Val (x), left (null), right (null) {}8  * };9  */Ten classSolution { One  Public: Avector<int> Preordertraversal (TreeNode *root) { -vector<int>result; -         if(!root)returnresult; theTreeNode *prev =NULL; -          while(root) { -Result.push_back (root->val); -             if(!root->Left ) { +root = root->Right ; -}Else { +Prev = root->Left ; A                  while(Prev->right && prev->right! = root) prev = prev->Right ; at                 if(!prev->Right ) { -Prev->right =Root; -root = root->Left ; -}Else { -Prev->right =NULL; -root = root->Right ; in Result.pop_back (); -                 } to             } +         } -         returnresult; the     } *};

Leetcode–refresh–binary Tree Pre Order traversal