LintCode: Binary Tree Postorder Traversal

C++，递归

 /**
  * Definition of TreeNode:
  * class TreeNode {
  * public:
  *     int val;
  *     TreeNode *left, *right;
  *     TreeNode(int val) {
  *         this->val = val;
  *         this->left = this->right = NULL;
  *     }
  * }
  */
 class Solution {
     /**
      * @param root: The root of binary tree.
      * @return: Postorder in vector which contains node values.
      */
 public:
     vector<int> postorderTraversal(TreeNode *root) {
         // write your code here
         vector<int> result;
         if (root == NULL) {
             return result;
         }
         if (root->left != NULL) {
             vector<int> left = postorderTraversal(root->left);
             result.reserve(result.size() + left.size());
             result.insert(result.end(), left.begin(), left.end());
         }
         if (root->right != NULL) {
             vector<int> right = postorderTraversal(root->right);
             result.reserve(result.size() + right.size());
             result.insert(result.end(), right.begin(), right.end());
         }
         result.push_back(root->val);
         return result;
     }
 };

C++，递归，辅助函数

 /**
  * Definition of TreeNode:
  * class TreeNode {
  * public:
  *     int val;
  *     TreeNode *left, *right;
  *     TreeNode(int val) {
  *         this->val = val;
  *         this->left = this->right = NULL;
  *     }
  * }
  */
 class Solution {
     /**
      * @param root: The root of binary tree.
      * @return: Postorder in vector which contains node values.
      */
 public:
     vector<int> postorderTraversal(TreeNode *root) {
         // write your code here
         vector<int> result;
         if (root == NULL) {
             return result;
         } else {
             postorderCore(root, result);
         }
         return result;
     }
     void postorderCore(TreeNode *root, vector<int> &result) {
         if (root == NULL) {
             return;
         }
         if (root->left != NULL) {
             postorderCore(root->left, result);
         }
         if (root->right != NULL) {
             postorderCore(root->right, result);
         }
         result.push_back(root->val);
         return;
     }
 };

C++，非递归

［一个stack］

［一个cur指针］

［一个pre指针］

 /**
  * Definition of TreeNode:
  * class TreeNode {
  * public:
  *     int val;
  *     TreeNode *left, *right;
  *     TreeNode(int val) {
  *         this->val = val;
  *         this->left = this->right = NULL;
  *     }
  * }
  */
 class Solution {
     /**
      * @param root: The root of binary tree.
      * @return: Postorder in vector which contains node values.
      */
 public:
     vector<int> postorderTraversal(TreeNode *root) {
         // write your code here
         vector<int> result;
         if (root == NULL) {
             return result;
         }

         TreeNode *cur = root, *pre = NULL;
         stack<TreeNode *> sta;

         while (cur != NULL || !sta.empty()) {
             while (cur != NULL) {
                 sta.push(cur);
                 cur = cur->left;
             }
             cur = sta.top();
             if (cur->right == NULL || cur->right == pre) {
                 sta.pop();
                 result.push_back(cur->val);
                 pre = cur;
                 cur = NULL;
             } else {
                 cur = cur->right;
             }
         }
         return result;
     }
 };