面試算法題. 二叉樹前、中、後序遍歷的迭代版本. 簡單複習

 // 1. 先序遍歷

 void FirstSearch(TreeNode* root) {

         if (nullptr == root)
                 return;

         stack<TreeNode*> stack_;
         stack_.push(root);

         while (stack_.size()) {

                 TreeNode* node = stack_.top();
                 stack_.pop();

                 // 處理當前節點
                 cout << node->val << endl;

                 if (nullptr != node->right) 
                         stack_.push(node->right);
                 if (nullptr != node->left)
                         stack_.push(node->left);
         }     
 } 

// 2. 中序遍歷
 void MiddleSearch(TreeNode* root) {

         if (nullptr == root)
                 return;

         stack<TreeNode*> stack_;
         stack_.push(root);

         while (stack_.size()) {

                 while (nullptr != root->left) {

                         stack_.push(root->left);
                         root = root->left;
                 }
                
                 while (stack_.size()) {

                         TreeNode* node = stack_.top();
                         stack_.pop();

                         // 處理節點
                         cout << node->val << endl;

                         if (nullptr != node->right) {

                                 stack_.push(node->right);
                                 root = node->right;
                                 break;
                         }
                 }
         }
 }

 // 3. 後序遍歷
 
 void BackSearch(TreeNode* root) {

         if (nullptr == root)
                 return;

         stack<TreeNode*> stack_;
         stack_.push(root);
         TreeNode* lastNode = nullptr;

         while (stack_.size()) {

                 while (nullptr != root->left) {

                         stack_.push(root->left);
                         root = root->left;
                 }

                 while (stack_.size()) {

                         if (lastNode == stack_.top()->right || nullptr == stack_.top()->right) {

                                 TreeNode* node = stack_.top();
                                 stack_.pop();
                                 cout << node->val << endl;
                                 lastNode = node;
                         }
                         else {
                                 root = stack_.top()->right;
                                 stack_.push(root);
                                 break;
                         }
                 }
         }
 }