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=0; i<n-1; i++) {
            allNodes[i]->left = NULL;
            allNodes[i]->right = allNodes[i+1];
        }
        allNodes[n-1]->left = allNodes[n-1]->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;
    }
};