Update binary-tree-preorder-traversal.cpp

C++/binary-tree-preorder-traversal.cpp

@@ -10,57 +10,19 @@
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
-class Solution {
-public:
-    vector<int> preorderTraversal(TreeNode* root) {
-        vector<int> res;
-        auto *curr = root;
-        while (curr) {
-            if (!curr->left) {
-                res.emplace_back(curr->val);
-                curr = curr->right;
-            } else {
-                auto *node = curr->left;
-                while (node->right && node->right != curr) {
-                    node = node->right;
-                }
-                if (!node->right) {
-                    res.emplace_back(curr->val);
-                    node->right = curr;
-                    curr = curr->left;
-                } else {
-                    node->right = nullptr;
-                    curr = curr->right;
-                }
-            }
+void traverse(TreeNode* root,vector<int> &temp){
+    if(root==NULL){
+            return;
         }
-        return res; 
-    }
-};
-
-// Time:  O(n)
-// Space: O(h)
-class Solution2 {
+    temp.push_back(root->val);
+    traverse(root->left,temp);
+    traverse(root->right,temp);
+}
+class Solution {
 public:
     vector<int> preorderTraversal(TreeNode* root) {
-        vector<int> res;
-        stack<pair<TreeNode *, bool>> s;
-        s.emplace(root, false);
-        while (!s.empty()) {
-            bool visited;
-            tie(root, visited) = s.top();
-            s.pop();
-            if (root == nullptr) {
-                continue;
-            }
-            if (visited) {
-                res.emplace_back(root->val);
-            } else {
-                s.emplace(root->right, false);
-                s.emplace(root->left, false);
-                s.emplace(root, true);
-            }
-        }
-        return res;
+        vector<int> ans;
+        traverse(root,ans);
+        return ans;
     }
 };