LintCode: Flatten Binary Tree to Linked List

C++

Traverse

 /**
  * Definition of TreeNode:
  * class TreeNode {
  * public:
  *     int val;
  *     TreeNode *left, *right;
  *     TreeNode(int val) {
  *         this->val = val;
  *         this->left = this->right = NULL;
  *     }
  * }
  */
 class Solution {
 public:
     /**
      * @param root: a TreeNode, the root of the binary tree
      * @return: nothing
      */
     void flatten(TreeNode *root) {
         if(!root) return;
         vector<TreeNode*> allNodes;
         preorder(root, allNodes);
         int n = allNodes.size();
         for(int i=; i<n-; i++) {
             allNodes[i]->left = NULL;
             allNodes[i]->right = allNodes[i+];
         }
         allNodes[n-]->left = allNodes[n-]->right = NULL;
     }

     void preorder(TreeNode *root, vector<TreeNode*> &allNodes) {
         if(!root) return;
         allNodes.push_back(root);
         preorder(root->left, allNodes);
         preorder(root->right, allNodes);
     }
 };

C++

Divide-Conquer

Recursive

 /**
  * Definition of TreeNode:
  * class TreeNode {
  * public:
  *     int val;
  *     TreeNode *left, *right;
  *     TreeNode(int val) {
  *         this->val = val;
  *         this->left = this->right = NULL;
  *     }
  * }
  */
 class Solution {
 public:
     /**
      * @param root: a TreeNode, the root of the binary tree
      * @return: nothing
      */
     void flatten(TreeNode *root) {
         helper(root);
     }

     TreeNode* helper(TreeNode *root) {
         if (root == NULL) {
             return NULL;
         }
         if (root->left == NULL && root->right == NULL) {
             return root;
         }
         if (root->left == NULL) {
             return helper(root->right);
         }
         if (root->right == NULL) {
             root->right = root->left;
             root->left = NULL;//!important
             return helper(root->right);
         }
         //Divide
         TreeNode *leftLastNode = helper(root->left);
         TreeNode *rightLastNode = helper(root->right);

         //Conquer
         leftLastNode->right = root->right;
         root->right = root->left;
         root->left = NULL;//!important
         return rightLastNode;
     }
 };